|
Geological Survey Professional Paper 3
The Geology and Petrography of Crater Lake National Park |
THE PETROGRAPHY OF CRATER LAKE NATIONAL PARK
By HORACE BUSHNELL PATTON.
HYPERSTHENE-DACITES.
(continued)
DISTRIBUTION AND DESCRIPTION OF DACITE MASSES.
LLAO ROCK FLOW.
As far as may be judged from the few specimens collected from this most conspicuous of the Crater Lake dacite flows, the Llao Rock mass shows three quite distinct varieties of dacite—namely, vitrophyric, lithoidal, and spherulitic dacite.
VITROPHYRIC DACITE.
This variety was collected from the extreme southern edge of the lava flow, where it is necessarily very thin. It is represented by Nos. 101 and 102. The hand specimens present the appearance of a perfectly fresh vitrophyre in which the glass base, which constitutes at least four-fifths of the entire mass, has a dark, greenish-gray color. In this glass base are inclosed numerous phenocrysts of plagioclase that measure not over 1 to 2 millimeters, and that are white and glassy and show hardly a trace of cleavage. In addition to these are a very few minute, deep-brown to green and black, resinous-lustered crystals that are hardly to be seen without a magnifying glass, and that, in thin section, prove to be usually hypersthene or hornblende. There are also to be noted a few angular fragments, compact, grayish brown and dull lustered, measuring one-quarter inch to one inch or more in diameter. These are referred to later under the head of nodular secretions. This rock is more or less cracked in different directions owing to shrinkage in cooling. The rock parting along these cracks, which sometimes gap, breaks into very smooth flat surfaces that have a distinct gloss. Otherwise the fracture is rather rough or small-concoidal, owing to the presence of the abundant plagioclase phenocrysts. This vitrophyric dacite may also be streaked by more or less parallel bands of grayish-looking pumice, as may be seen in a specimen collected by the writer on the same spot. (No. 2013.1 of collection of H. B. Patton.)
In thin section the glass base appears to be clear and colorless and to be crowded with very sharp and straight and remarkably even-sized microlites of augite that measure 0.003 millimeter wide by 0.02 to 0.04 millimeter long. (See fig. A of Pl. XVIII, p. 132.) These microlites are too small to show any color, but the strong refraction is evident, as well as double refraction and an extinction angle up to 45° are also to be seen a very few opaque curved black trichites. The straight, colorless augite microlites appear to be the same as are to be seen sparingly developed in a thin section prepared by the United States Geological Survey from the rhyolitic perlite of the Yellowstone National Park.a They still more closely resemble in size, appearance, and numbers the straight colorless microlites in the dacite from Lassen Peak, California, No. 82 of the above-mentioned Educational Series.b The close resemblance between the Llao Rock dacite and this dacite from Lassen Peak will be again referred to in these pages.
aNo. 61 of the Educational series of Rocks, described by J. P. Iddings in Bull. U. S. Geol. Survey No. 150, p. 151.
bJ. S. Diller, op. cit., p. 217.
A few feldspar microlites occur in No. 101, but almost none in No. 102. These are in part lath-shaped plagioclase with very small extinction angles, but mainly short rectangular to square, untwinned feldspar that usually extinguishes nearly parallel to the sides. In No. 101 was seen a nearly rectangular section of feldspar that was not larger than nor even as large as many of the feldspar microlites (0.05 millimeter long), in the center of which was a brown-glass inclusion with sides parallel to the crystal edges and occupying at least one-third of the whole crystal, itself inclosing a comparatively large air bubble. The extinction angle measured to the longest side was 18° twinning was apparent. A reproduction of this crystal may be seen in fig. G of Pl. XIV, p. 76.
Fluidal structure, which is hardly to be seen in the hand specimen, is very conspicuous in the thin section, owing to the more or less parallel arrangement of the microlites which occur in flowing lines lapping around the phenocrysts of plagioclase, hypersthene and hornblende, and of accessory magnetite and apatite. Hypersthene is scarce and hornblende still more so. The latter occurs mainly in very small needles and only occasionally in crystals comparable in size with the hypersthene. In one instance a small crystal of hornblende with sharp crystal forms was observed clearly inclosed in plagioclase. This is a decided exception to the rule that hornblende in these dacites is the youngest of the phenocrysts with the possible exception of augite.
The chemical analysis of No. 102 is given on page 140.
LITHOIDAL DACITE.
This rock (103) comes from the summit of Llao Rock. It consists of a light gray, dense, and somewhat porous groundmass with thickly scattered, small, white feldspar phenocrysts, similar to those in the vitrophyric variety: it consists also of a few almost unnoticeable darker phenocrysts. It breaks with a decidedly rough fracture and does not show any fluidal structure.
In thin section the groundmass is seen to contain small plagioclase laths, similar to, but much more abundant than those mentioned as occurring in No. 101; also a few feldspar crystallites with rectangular outlines and much more feldspathic matter that shows no distinct forms and that is not capable of absolute identification. There is undoubtedly much glass present but, being colorless, it is almost concealed by the abundant crystalline matter inclosed. In white light with weak magnifying powers the thin section has a somewhat dusty appearance. When strongly magnified this dusty matter resolves itself into minute, colorless, straight angite microlites, like those described as characteristic of the vitrophyric rhyolite, together with somewhat larger microlites of the same mineral that inclose a few very minute, black, and opaque grains—presumably of magnetite. Probably the dusty appearance is intensified by the presence of small air bubbles in the glass base. Phenocrysts occur about as in Nos. 102 and 101, with the addition of a very little augite in sharply defined and almost colorless crystals.
This can not be said to be a typical lithoidal dacite. The abundance of plagioclase microlites and the scarcity of the untwinned groundmass feldspar with square or rectangular forms cause this rock to resemble some of the more dacitic andesites of this region.
SPHERULITIC DACITE.
A specimen of spherulitic dacite, No. 108, was collected on Llao Rock in 1883. The field label accompanying the specimen does not state the exact location. In the hand specimen this is seen to consist of a nearly black glass very thickly crowded with brownish spherulites that measure from 3 to 5 millimeters in diameter. So thickly crowded are these spherulites that they often interfere and make up much more than one-half of the mass. The spherulites consist of two, sometimes three parts. The center is of a dark-gray color, and has a very dense, felsitic texture. Around this is a ring or zone about a millimeter wide, of a less dense or even of minutely porous material, and of a brownish, reddish-brown, or light grayish-brown color. Outside of this again occurs usually, but not always, the third part, dense like the central portion, and either gray in color like the center or of a deeper brown than the intermediate zone. Many of the spherulites show gapping cracks that seem to be confined to the middle zone. There is, however, no trace of lithophysal development. The inside of these cavities is rough, and of about the same color as the middle zone of the spherulites in which they lie. They do not appear to contain crystals. Upon cross fracture these spherulites have a distinct radiated appearance. The customary phenocrysts occur quite indifferently in the glassy portion as well as in the spherulites.
Under the microscope the glassy groundmass is almost identically the same as that of the vitrophyric dacite described above. The same colorless augite microlites of about the same size, 0.03 to 0.05 millimeter long by 0.003 millimeter wide, may be seen. A very slight distinction may be noted in that these augite microlites are not perfectly clear, but often have a small amount of black, opaque, dusty matter—probably magnetite—either adhering to the outside or inclosed within. Curved, black microlites were not observed. Most of the spherulites show two or three periods of growth that correspond to the different colored zones noticeable in the hand specimen. One or two appear to have had but one period of growth—that corresponding to the inner portion of the others. This inner part has a dirty-brown or grayish-brown color, lets through but little light, and shows a distinctly fibrous radiating structure. The fibers are very fine, and hardly distinguishable from each other. They polarize light feebly, and have a positive extension. Owing to the partial opacity of this central portion, the usual black cross is hardly discernible.
The portion of the spherulites that belongs to the second or intermediate zone does not seem to be as porous as the rather rough appearance in the hand specimen would indicate. It is, in fact, mostly quite solid. It appears in a light-brown color, very much lighter than the central part, and is composed essentially of distinct shreds of a colorless mineral diverging from the center outward and branching at low angles. These shreds are coarse enough to show extinction angles often quite oblique to their longer axes. They have invariably a positive extension. In polarized light they appear to continue the finer fibers of the central portion. Between the arms of the branching positive shreds occurs frequently unindividualized matter that shows feebly negative polarization—considering this substance to be also fibrous; but all the more distinctly recognizable shreds are positive.
These coarse branching shreds so closely correspond to the feldspar of the spherulites in the obsidian of Obsidian Cliff in the Yellowstone National Park, as described by Iddings,a and to that of the spherulites from the region of Rosita and Silver Cliff, Custer County, Colo., as described by Cross,b that, after studying thin sections prepared from spherulites from the latter place, the writer has no hesitancy in pronouncing these in the Llao Rock dacite as belonging to feldspar also. In their positive character they correspond to the feldspar in many of the spherulites from Custer County, Colo. The brownish color of this intermediate portion is due to the presence of brown, yellowish, and reddish ferritic matter in the form of dust particles, and occasionally in the form of minute scales. A radial arrangement of this ferritic matter is not marked.
aSeventh Ann. Rept. U. S. Geol. Survey, 1888, pp. 276-278.
bConstitution and origin of spherulites in acid eruptive rocks: Bull. Philos. Soc. Washington, vol. XI, 1891 pp. 411-440.
The outer portion of these spherulites when seen in thin sections appears to be identical with the central core. It is often entirely missing, and when present does not usually envelop the whole spherulite, but appears as irregular lobes or prolongations of the same.
The strings of augite microlites that accentuate the fluidal structure of the glassy part of the rock pass uninterruptedly through these spherulites, and the phenocrysts lie embedded in them as well as in the remainder of the rock, as is universally the case in such bodies. Part of the ferritic matter may be seen to arise from the further oxidation or hydration of the ore particles that adhere to the augite microlites.
The phenocrysts observed in this rock are plagioclase hypersthene, hornblende and augite, with accessory apatite and magnetite. The order of crystallization is (1) magnetite and apatite, (2) plagioclase, (3) hypersthene, (4) augite and hornblende, (5) the spherulitic forms; to which may be added augite and feldspar microlites of the groundmass, which belong between 4 and 5. The hornblende is very sparingly developed, as is usual, but occurs in both the brownish-green and in the brownish-red varieties.
This spherulitic dacite should be compared with the very similar occurrences in the Cloud Cap flow.
in the thin section of this rock may also be seen a few small inclusions of identical nature with the inclusions of older secretions to be found in No. 102, described immediately below.
NODULAR SECRETIONS.
In the description of the vitrophyric dacite (102) from the south end of the Llao Rock flow, reference is made to inclosed angular fragments of compact, grayish-brown, dull-lustered material. In the thin section of this rock there occur three of these inclusions, the largest of which is about 6 millimeters in diameter. They are composed of a loosely felted mass of slender hornblende prisms and also of almost equally slender plagioclase laths, with a few octahedrons of magnetite. In the interstices of this felt is to be seen a brown glass that composes at most one-quarter of the mass. The hornblende and plagioclase are present in about equal amounts. The hornblende is very uniformly about 0.1 millimeter in width and from 0.5 to 1 millimeter in length; the plagioclase is about the same in width, but not quite so long. In one of these inclusions the color and pleochroism of the hornblende are almost identically the same as that of the reddish-brown hornblende that is mentioned above as occurring in many of the dacites. It is to be distinguished only by the very slender form and by its great abundance. The extinction angles in the prism zone are very small, rarely over 3°.
In another of these inclusions the hornblende has the color and pleochroism of the greenish-brown hornblende of the dacites, while in the third one the color is intermediate between the two. This change of color, corresponding as it does to the variations of color of the hornblende in the different dacites or in the same dacite, forms one of the arguments in favor of this being an older secretion. The form of this hornblende does not vary with the change of color. Usually only the unit prism is to be noted. The terminations are not often sharp. The prisms either taper out at the end, or end roughly, as though broken off.
The plagioclase is usually simply twinned, with only two or three bands visible. The largest extinction in a symmetrical section observed was 30° very slender laths contain long brown glass inclosures that often show the form of the host. The prisms of plagioclase and hornblende do not often interfere, but when they do it does not appear that one of these ingredients is older than the other. In addition to these lath-shaped plagioclase, this mineral also occurs in two or three comparatively large and decidedly spongiform, stout crystals that contain much colorless glass, and bits of hornblende of the same color as in the section outside of the feldspar. It is rather remarkable that this mineral should be thus inclosed in the plagioclase, inasmuch his elsewhere in these dacites hornblende belongs to the youngest of the phenocrysts. It is to be noted, however, that, as it occurs thus inclosed in plagioclase, it does not have the same sharp form as is otherwise to be seen. In fact, it presents exactly the appearance of having been formed as a secondary mineral in the feldspar.
Both hypersthene and augite seem to be missing.
There does not appear to the writer to be much doubt that these inclosures are fragments of older secretions from the dacitic magma, although the absence of the pyroxenes is hard to explain on this supposition. But the tendency for the ingredients to assume long slender forms, quite distinct from those in the rock in which they occur, appears to be very characteristic not only of this but of other secretions that will be described later as occurring in the dacites, and also of secretions in the andesites.
Fig. A of Pl. XVIII (p. 132) is a photomicrograph from No. 102, and shows both the secretion and the inclosing vitrophyric dacite.
Reference is made elsewhere to the resemblance between some of the vitrophyric dacites of Crater Lake and the dacite from Lassen Peak, California, of which Mr. Diller has given a brief description.a In this description Mr. Diller mentions and gives a photograph of angular nodules inclosed in the dacite. His description of the microscopic appearance of these inclusions, corroborated by study of the thin sections kindly loaned the writer for the purpose, discloses a very close resemblance to the inclusions from the Llao Rock dacite. In color, form, and structural relationships of the hornblende and in the occurrence of plagioclase and a small amount of brown glass the resemblance is very close. The main points of difference are these, that the Lassen Peak inclusion is coarser grained, the hornblende prisms are not so slender, and the hornblende not infrequently incloses plagioclase laths; also biotite amid pyroxene and a little olivine and tridymite are present. The appearance of biotite in the Lassen Peak secretion is to be expected, as the same mineral occurs in the dacite itself.
aBull. U. S. Geol. Survey No, 150, 1898, p. 217.
GROUSE HILL FLOW.
The four specimens collected from this mass (105, 106, 107, and 108) are far from being a typical dacite. If this were the only example of this kind of rock in connection with the Crater Lake lavas the rock would naturally be classified as an andesite. As a matter of fact it partakes of the characteristics of both dacites and andesites to such an extent as to represent a thoroughly intermediate type. Taken in connection with both the andesites and the dacites of the surrounding region the dacitic characteristics predominate, so that the rock may well be called an andesitic dacite.
These four specimens are light gray or, in the case of No. 107, brown and gray streaked, rough-fracturing and slightly porous rocks with the usual small find not very conspicuous phenocrysts. Under the microscope the groundmass varies from glassy to hypocrystalline, the variations occurring in irregular streaks to be seen in the same thin section. In general the groundmass contains a good deal of crystalline matter, which consists mainly of small lath-like microlites of plagioclase, together with small, rectangular, untwinned feldspar crystallites, such as are to be found very abundantly in the more typical dacites. The plagioclase microlites are similar to those seen in the dacite of Llao Rock, but are here much more numerous and are the feature that most closely reminds one of the andesites. They are probably oligoclase, as the extinction angles are mostly very small. In addition to these feldspars may also be seen very slender and minute colorless microlites resembling the augite microlites of the Llao Rock flow, but differing in being not quite so sharp or straight, also in having a tendency to taper out at the end. They are undoubtedly augite. A small amount of black, opaque, iron oxide, distributed as minute, dust-like particles, assists the augite microlites in giving to the groundmass a clouded aspect. The brown color of No. 107 is produced by the presence of brownish and yellowish ferritic staining matter.
All four of these specimens contain plagioclase, hypersthene, augite, and reddish-brown hornblende as phenocrysts. The plagioclase has a stronger tendency than is customary in the Crater Lake dacites to become crowded with glass inclusions. Augite is more abundant than in the more typical dacites. It occurs both in sharp but small and often twinned crystals and in irregular grains. The hornblende, which belongs to the reddish-brown variety, is also somewhat more abundant than usual. In No. 106 it occurs in uncorroded crystals, on one of which the forms (110), (010), and (100) were noticed. In the others the hornblende is more or less corroded and has developed resorption rims of magnetite and augite. In Fig. H of Pl. XIV (p. 76) may be seen a reproduction of a crystal of hornblende in No. 105.
CLEETWOOD COVE FLOW.
This mass is one of the latest lava flows to be found on the rim of the crater. According to Mr. Diller a part of this stream was fluent at the time the summit of Mount Mazama subsided, and a portion of the stream that ran back into the pit formed by the subsiding cone may be seen at the head of the cove at the water's edge. The lava of this mass, as far as may be judged from the five specimens collected, is of two varieties, a vitrophyric and slightly spherulitic and a lithoidal variety.
The vitrophyric type is seen in Nos. 109, 110, and 111, which were collected on the crest of the rim. These appear in the hand specimens to be very glassy and to consist of mixtures of black to grayish-black and of reddish-brown glass. The reddish-brown portion is a little more abundant than the black and forms more or less continuous streaks, while the black is more frequently in the form of small, angular patches, inclosed in the former. When examined with a pocket magnifying glass the brown portions appear often porous and possess a somewhat ropy structure, while the black portions are solid. Small phenocrysts of white glassy-looking plagioclase abound as usual.
In thin section it is evident that the brownish portions result from the alteration of the black glass simply by a process of hydration of the iron contained in the glass. The fresh and unaltered parts appear in a thin section as a light-brown glass, perfectly fresh and clear, but irregularly dotted with very small colorless glass spots. This light-brown glass contains countless minute and colorless augite microlites which, in No. 109, are identically like those in the glassy dacite from the southern end of Llao Rock. In No. 111 the groundmass contains, in addition to these augite microlites, also a few plagioclase microlites. In No. 110 the augite microlites are more slender and longer, as well as more numerous, and the feldspar microlites occur in lath-shaped plagioclase, and still more conspicuously in the squarish or rectangular untwinned forms that are more especially characteristic of dacites. In all three of these there is a partial development of spherulitic forms. These occur in small reddish to brownish or yellowish spheres that very faintly polarize light and have a not very distinct radial structure.
The brown color of the rock as seen in the hand specimen is due largely to the ferritic matter contained in the spherulites, but is also due to staining of the glass in the vicinity of cracks and pores and around the spherulites.
The phenocrysts, with the exception of plagioclase, are small and very scarce. They consist of hypersthene and a very few minute, reddish-brown hornblendes (not more than three or four in a thin section). Augite is entirely wanting.
The lithoidal variety of dacite from this lava flow is seen in Nos. 112 and 113. The former has a dark-gray, compact, and dull-lustered groundmass and strongly developed fluidal structure; the latter is light gray and lusterless, but not fluidally developed. Both of these probably contain considerable glass, but in No. 112 this is partially, and in No. 113 almost entirely, obscured by devitrification products common to lithoidal dacites. In both, the augite microlites abound, but in No. 113 these are notably larger than in the other dacites heretofore described in these pages, and they are granulated by means of adhering and inclosed magnetite grains. In this rock, too, the colorless crystalline material of the groundmass is coarser than usual. Very little of it can actually be identified as plagioclase microlites. It consists, rather, of allotriomorphic shreds that may be feldspar or quartz, and that have a general elongation parallel to the fluidal planes of the rock. Small plagioclase phenocrysts abound, but other phenocrysts are very scarce. Hornblende is nearly missing and augite entirely so.
A chemical analysis of No. 113 will be found on page 140.
WINEGLASS FLOW.
This embraces only the small dacite flow, about three-quarters of a mile long, that starts near the rim at the Wineglass and extends northeastward. In the hand specimen the rock (114) closely resembles the brown and black vitrophyric dacite from the Cleetwood Cove region, just above described. In this case, however, the black glassy portions appear more as elongated streaks accentuating the fluidal structure.
Under the microscope the darker portions that appear black in the hand specimen are seen to be composed of nearly pure glass, but not of a uniform color. The rock consists of more or less parallel strands of absolutely colorless glass and of glass rendered more or less opaque by means of blackish to brownish minutest dust-like particles of uncertain character. The usual microlites of augite appear to be missing. This interweaving of colorless and dark-colored strands causes the glass to assume a decided stringy appearance. In case two phenocrysts approach each other the strings or strands are apt to assume almost or entirely parallel directions, but elsewhere they show a constantly varying arrangement, frequently becoming wavy or strongly crinkled, like the crinkling of many chlorite and mica schists. Furthermore, this stringy structure does not run uninterruptedly through the thin section, but is broken up into more or less separate areas that have lenticular or twisted forms. In general the arrangement of these glassy strands corresponds with the customary fluidal structure of glassy rhyolites and dacites, in that the strands lap or flow around the phenocrysts, after the manner of strings of microlitic inclosures.
The red portion of this dacite does not appear to differ in thin section from the above-described black areas, except that it is stained with dirty looking, brownish, powdery, ferritic matter. It constitutes the greater part of the rock and appears to inclose irregular, frayed out areas of the darker colored glass. This partial alternation of different colored areas lends a brecciated appearance to the whole.
The characteristic structure of this dacite does not differ essentially from similar structures as described by observers elsewhere, but it reminds the writer forcibly of the red obsidian which Iddings has described, and of which he has given a colored reproduction.a The phenocrysts of this dacite are not noticeably different from those in the dacites already described. They consist of plagioclase and hypersthene and of isolated, minute, reddish-brown hornblende. The rock also contains small fragments of a holocrystalline, porphyritic, hypersthene-bearing basalt, and in one case at least of a hyalopilitic andesite. These fragments are with a few exceptions hardly noticeable in the hand specimen, as they are small, but they are very numerous, as each of the two thin sections prepared from this rock contains eight or nine such inclosures that measure from 2 or 3 millimeters down to about 0.02 millimeter in diameter.
aSeventh Ann. Rept. U. S. Geol. Survey, 1886, p. 1.
The chemical analysis of this dacite will be found on page 140. It will be noted that the amount of silica is 2 or 3 per cent less than in the two other dacites analyzed. Unfortunately the presence of the above-mentioned inclosures of basalt vitiate to some extent the analysis. The lower percentage in silica may easily be explained by the presence of these inclosures, which may have been present in the portion sent for analysis to an even greater extent than is indicated by the thin sections.
CLOUD CAP FLOW.
This is an extensive flow that starts at Cloud Cap and extends for about 3 miles in a northeasterly direction with a breadth of about a mile. The rock specimens collected from this flow and included in this description are Nos. 115, 116, 117, 118, 119, and 120. With the exception of one distinctly lithoidal specimen (119), these are mainly very glassy and also largely spherulitic dacites, closely resembling the Llao Rock mass (as seen in Nos. 101, 102, and 104). The glassy portion of these rocks has a light-gray color. Phenocrysts of white glassy plagioclase arc abundant, and phenocrysts of the ferromagnesian minerals are scarce. Spherulites are to be found in Nos. 116, 117, and 120, but they are conspicuously visible in the hand specimen only of the last mentioned number. Here they occur from one-fourth to one-half inch in diameter, and have a whitish to light-drab color. The larger spherulites are more or less hollow and show on the cracked surface a fluted or ribbed structure radiating from the center outward.
As this rock does not differ very materially from the spherulitic dacites already described, a brief description of the microscopic characters will suffice for present purposes. The glassy portions in the spherulitic varieties, as well as in the entirely vitrophyric groundmass of No. 118, contain characteristic streams of minute straight and colorless augite microlites, like those in Nos. 101 and 102 of the Llao Rock mass. Microlitic feldspars, in the form of lath-like plagioclase and rectangular and apparently untwinned feldspar, occur in varying amounts, both in the same and in different thin sections. Some streaks may be almost entirely devoid of such feldspar microlites, while others may be crowded thickly with them, while between these two extremes there appears every conceivable intermediate stage. In two or three thin sections but more noticeably in No. 117, the untwinned feldspar, or more properly the microlitic feldspar, that does not appear to have the albitic twinning, occurs in very thin leaves that show striking Carlsbad twinning. The two individuals that make up one of these twins have each of them simple quadrilateral form, and the leaves lie over each other joined by the clinopinacoid, which is also the plane parallel to which the little leaves are extended. Professor Rosenbusch refers to such twinned microlitic feldspars as occurring in rhyolite pitchstone and rhyolite obsidian. No. 116, which shows in the hand specimen very strongly developed fluidal structure, consists of alternating, parallel streaks of vitrophyric, spherulitic dacite, and of a more lithoidal dacite free from spherulitic inclusions. The lithoidal streaks have the plagioclase laths very strongly developed and at times bear a marked resemblance to some of the Crater Lake andesitic rocks. On the other hand, No. 115 has a brown glass groundmass, inclosing numberless somewhat granulated augite microlites, and is almost free from feldspar microlites of any description. Slender, straight, and curved opaque black trichites, as well as a very little black ore in minutest grains, may also be mentioned as occurring in some of these specimens.
The spherulites differ somewhat from those in No. 104, in that zones of growth are not so plainly developed. There is, however, often an inner, coarse-grained portion occupying the greater part of the spherulite and an outer, denser, and usually deeper stained zone. The inner coarser part is made up of radiating shreds that show both positive and negative extinctions; usually, however, positive. They also have a tendency to fork at small angles and do not extinguish always parallel.
The phenocrysts do not present any characteristics peculiarly different from those common to the other dacites with the possible exception of hornblende, which is rather more abundant than elsewhere. It has mostly a greenish-brown color, but also occurs reddish-brown in No. 116. It is absent only in No. 115. It is of interest to note in this connection that, although these rocks contain more hornblende than do the other dacites of Crater Lake, still the actual amount is very small indeed. As far as may be judged, a thin section from this flow does not contain one-fiftieth or perhaps one-hundredth part as much hornblende as does the dacite from Lassen Peak, to which reference is made above and with which these Crater Lake rhyolites are closely allied, it should further be stated that occasional nests of older secretions are to be seen consisting of plagioclase, hypersthene, hornblende, augite, and magnetite, in which hornblende is apt to be very abundant. Zircon, which is so often reported as occurring in such rocks, appears to be a rare accessory mineral in the Crater Lake rocks of all types. It was noticed in a single crystal in No. 117.
SUN CREEK FLOW.
This mass includes all the dacitic area to be seen on both sides of Sun Creek, and stretching to the east as far as Sand Creek. The specimens studied are Nos. 121, 122, 123, 124, 125, 126, 127. No. 144 comes from a secretion in dacite and will be described later. No. 123, which is distinctly different from the others, will be found described on page 139, under the head of secretions. No. 128 is a dacite tuff.
This is the only dacite mass to be seen on the south side of Mount Mazama. It differs from all the above-described dacites, in that, as Mr. Diller has shown, it belongs to an earlier period in the formation of the volcano and is overlain by andesite flows. Although the individual specimens above numerated vary greatly in their outward appearance, they are in general much more thoroughly crystalline than are the more recent dacites on the northern and eastern sides of the lake. A glassy groundmass is not wanting in portions of these rocks, but devitrification, either contemporaneous with the cooling of the rock or subsequent thereto, is well developed. These rocks, therefore, are better suited to a study of the lithoidal types and will receive more individual attention than has been given to most of the specimens thus far.
No. 121, taken from the west side of Sun Creek Canyon, is a light-gray, porous, and rough-fracturing rock, with inconspicuous phenocrysts, and with numerous rough cavities that are lined with minute white glassy crystals. Some of these crystals, scraped from the cavity with a knife, proved to be tridymite, together with some feldspar similar to that described later as visible in the thin section lining the cavities.
Under the microscope the groundmass appears to be in most places holocrystalline, or at least one in which a glass base plays a very subordinate role. Here and there, however, the thin section assumes a brownish cast, indicative of abundant glass. The colorless particles, which appear to make up the bulk of the groundmass, are neither elongated nor squarish in cross section, but have ill-defined, roundish, allotriomorphic form. Throughout this granular mass are scattered minute opaque dust-like ore particles, and also many minute yellowish to colorless dust-laden augite microlites. Irregular longish cavities abound, the larger of which are empty; the smaller ones are often filled with tridymite. Between this tridymite and the walls of the cavity is a narrow fringe of colorless grains, the same to all appearances as those that form a large part of the groundmass. Wherever these colorless grains come into contact with the Canada balsam they are seen to have an index of refraction lower, but only very slightly lower, than that of the balsam, so that the edge of the grain can hardly be distinguished. They can not, therefore, be quartz. They are structureless and without twinning bands. In polarized light the polarization colors are much higher than in the adjacent tridymite. Some of the largest of these grains (0.02 to 0.03 millimeter) give biaxial images in convergent polarized light. In one case a distinct cleavage was noticed with an extinction angle of 7-1/2° trace of the cleavage plane. Convergent polarized light gave a positive large-angled bisectrix in the center of the field, with the plane of the optical axes inclined 7-1/2° trace of the cleavage. These properties would certainly indicate orthoclase, in which case the grain just mentioned is cut nearly parallel to the clinopinacoid. These fringing grains are free from solid inclosures, but they appear to contain gas pores.
That, in spite of the above-described properties, these colorless grains are probably not orthoclase, may appear from the following two considerations: First, the index of refraction is too high for orthoclase, although, like orthoclase, it is lower than that of the Canada balsam, whose index of refraction is taken to be 1.540. Orthoclase, whose index of refraction is about 0.025 less than that of balsam, should present a sharply defined edge at contact with the balsam. Second, many or even most of these grains have an undulous extinction. This second consideration would suggest the presence of anorthoclase, which would also be corroborated by the preponderance of Na2O over K2O, as shown in the chemical analyses of the Crater Lake dacites. On the other hand, the index of refraction of anorthoclase as given by Professor Rosenbuscha is only 1.527, which is but 0.002 higher than that of orthoclase, Unless, therefore, anorthoclase should prove to possess a higher index of refraction than that here given it would seem that this feldspar must be placed in the albite-oligoclase series in spite of the fact that no twinning is to be seen.
aMikroskopische Physiographie, 3d edition, Vol. I, 1892, p. 157.
It is more than likely that quartz forms a large part of the crystalline groundmass, but owing to the small size of the particles this could not be demonstrated.
The tridymite of this rock does not possess the customary shingled aspect. It does probably occur in more or less overlapping scales, but the edges are irregular and the overlapping not very conspicuous. The occurrence, however, is exactly like that of the tridymite in the lithoidal rhyolite from Obsidian Cliff in the Yellowstone National Park, as examined by the writer in sections from that rock in the possession of the mineralogical laboratory of Harvard University. In describing the occurrence of tridymite in the rhyolite from Obsidian Cliff, Iddings says:b "The spaces * * * are in most instances occupied by tridymite in comparatively large crystals, often twinned and carrying numerous gas cavities." The tridymite in this Sun Creek dacite may be recognized by the following properties. It occurs in minute crystals or grains that measure 0.05 to 0.15 millimeter, usually filling the cavities. Twinning is very common. Sometimes the twins resemble those of orthoclase in the form of Carlsbad twins, the twinned halves having widely differing extinction angles; in other eases there appear to be twinning planes in more than one direction. Undulous extinction is very common, but where the twinning is well marked this is not much in evidence. Exactly similar undulous extinction was also noted in the sections from Obsidian Cliff above referred to. Interference colors are very low, sometimes being almost invisible. The refraction is much lower than that of the adjacent balsam. This great difference in refractive powers is strongly brought out by the extreme roughness of the tridymite, as well as by the sharpness of the edges as compared with the great smoothness of the surface of the feldspar grains fringing the cavities. Except for the presence of gas cavities. the tridymite is free from inclosures.
bSpherulitic crystallization: Bull. Philos. Soc. Washington. vol. XI, 1891, pp. 445-464.
Among the phenocrysts of this rock are fairly abundant plagioclase, rather sparingly developed hypersthene, and only an occasional minute augite crystal. Hornblende was not to be seen.
Specimen No. 122, collected lower down on the west side of Sun Creek Canyon, not far from No. 121, is a slightly porous light-gray rock of lithoidal character, mixed to a certain extent locally with dark grayish-green glassy portions. Both the lithoidal and glassy portions are crowded with small spherulites, 1 to 3 millimeters in diameter. Most of these spherulites are solid externally, and of a dark-gray color within on the freshly fractured face. Some are hollow and show a slight tendency toward the formation of concentric shells or lithophysae.
Under the microscope the greater part of the rock is seen to be composed of spherulites of distinctly recognizable fibers and shreds that radiate in branching, feathery forms from common centers. These apparently feldspathic fibers have both positive and negative, but more commonly positive, extension, and extinguish both parallel and oblique. Whether they are actually intergrown with quartz can not positively be asserted. The outer portion of the spherulite usually contains deep brownish-red scales, presumable of hematite, that polarize light strongly. They radiate from the center and branch at low angles. To these brownish-red scales is due the reddish color of the outer portion of the spherulites as seen in the hand specimen. The two thin sections prepared from this rock were both cut from the lithoidal part of the hand specimen, so that they do not show any glass under the microscope. The groundmass outside of the spherulites seems to be holocrystalline, but extremely fine grained. The individual grains are too fine to be determined, but they appear to have no particular elongation. Through this minutely granulated groundmass, as well as through the spherulites, run streams of slender, rod-like augite microlites and of minute but not very abundant black ore particles, to which should be added an occasional lath-shaped plagioclase microlite. Phenocrysts occur the same as in No. 121.
No. 124 is an aggregate of very light-gray, porous, and distinctly crystalline, as well as of very dark-gray, dense parts. In the thin section the granular parts have mostly disappeared in the grinding, but from what is left they seem to be composed of an aggregate of colorless, allotriomorphic grains, probably of feldspar and quartz, with occasional tridymite scales. The dense, darker-colored parts probably contain a glass base completely obscured by the devitrification products, which consist of augite microlites granulated with adhering and inclosed opaque ore particles and of extremely minute, colorless allotriomorphic shreds and grains. As in No. 122, this rock also contains a very few lath-shaped plagioclase microlites.
The phenocrysts consist of the customary plagioclase, with also a little hypersthene and two or three fragments of brown hornblende with black resorption rims.
No. 125, from near the head of the West Fork of Sand Creek, is somewhat similar to the last-described specimen, but the groundmass is much coarser grained. The shredded appearance of the colorless ingredients is very marked, hut their exact mineralogical nature can not be made out. These shreds do not reach dimensions greater than 0.05 millimeter, and are too small to clearly disclose twinning striation, although the larger ones leave the impression that polysynthetic twinning is present. In white light the granulated augite microlites show up very clearly and abundantly, even with moderate magnifying powers. Plagioclase and hypersthene are the only phenocrysts noted. This rock hears a close resemblance to No. 113 from the Cleetwood Cove mass.
No. 126, which was collected close to No. 125, has a very dense structure and dark-gray color. It looks decidedly like an andesite. In thin section it does not appear altogether even grained, but discloses coarser-grained patches. It is, however, as a rule, very fine grained, or, at least, appears so in white light, in which it appears to be composed of a uniform white substance very thickly sprinkled with opaque black dust particles. When strongly magnified these dust particles appear as black ore grains, and also as transparent or translucent globulitic matter. The granulated augite microlites so common in other specimens from this lava mass are almost absent. In polarized light the continuous white substance appears to be made of very irregular, interweaving allotriomorphic patches that remind one forcibly of the allotriomorphic feldspar patches of many of the holocrystalline andesites of this region. They are to be distinguished from such by the absence of inclosed plagioclase laths. Plagioclase and hypersthene occur among the phenocrysts, as usual. One augite crystal was noticed; also a grain of magnetite surrounded with a rim of leucoxene.
No. 127, from near Sand Creek, furnishes a beautiful illustration of fluidal structure, both in the hand specimen and in the thin section. In the former it appears in very thin alternating parallel bands of light reddish-gray and of dark-gray and very compact materials. Phenocrysts are small, but quite abundant, and are made more conspicuous by the evident flowing of these thin bands around them.
In thin section the fluidal structure is beautifully brought out, both by the alternation of coarser, white bands with finer-grained and brownish-colored bands and by the presence, especially in the finer-grained portions, of streaks of black dust. When examined with strong powers this apparent ore dust is seen to be composed in part of inclusions of air; in part also of really opaque particles that are usually to be seen inclosed in augite microlites. The white bands appear to be entirely crystalline. They contain comparatively little microlitic material, either in the shape of augite or ore particles. In polarized light the colorless material breaks up into very distinct, longish shreds quite similar to those described above in other specimens from this dacite mass. They are, perhaps, somewhat more distinct here than elsewhere. Occasionally some of the larger and better-defined shreds (0.1 millimeter or more in length) show distinct twinning bands, but this is exceptional. More frequently the extinction is more or less undulous and sometimes markedly so. In general it is nearly parallel to the extension and is always negative.
These shred-like strips show a strong tendency toward grouping themselves around common centers. Three, four, or five or more of these may be seen diverging from a common point, each of them narrowing down wedge-like at the center. As in the case of the more isolated shreds these are also negative. They thus produce what Rosenbusch calls "pseudospherulites" with negative crosses. In the midst of these "pseudospherulites" may also be seen the ordinary "true spherulites" of Rosenbusch, also with negative crosses. These latter do not appear to be composed of any recognizable mineral species. They show the customary black cross with arms parallel to the principal planes of the nicol prisms and remain stationary upon revolving the stage. They do not succeed in developing externally spherical forms, as their growth appears to be interfered with by contact with the coarser-grained portions of the groundmass. Every possible gradation appears to present itself between these so-called "true spherulites" and the radial groups of crystal particles that appear to be undoubtedly feldspathic.
The finer-grained bands with brownish color are composed, apparently, of glass with inclosed dust particles. But upon closer inspection the apparent glass seems to be composed of ill-defined particles that, owing to their parallel arrangement, very faintly polarize light, so that the whole band extinguishes parallel, with negative extensions. This is undoubtedly the substance so frequently referred to in describing such rocks as microfelsite. On each side of these uniformly polarizing bands of microfelsite occurs a very narrow, continuous spherulitic streak, the individual spherulitic parts of which are quite similar to the above-mentioned "true" sphemaulites.
The phenocrysts in this rock are similar to those in the rest of these dacites, namely, plagioclase and hypersthene. Hornblende could not be seen.
| <<< Previous | <<< Contents >>> | Next >>> |
pp/3/part2-2a.htm
Last Updated: 07-Mar-2006