HOT SPRINGS Analyses of the Waters of The Hot Springs of Arkansas Geological Sketch of Hot Springs, Arkansas

GEOLOGICAL SKETCH OF THE HOT SPRINGS DISTRICT, ARKANSAS.

The rocks seen about the Hot Springs are chiefly of sedimentary origin and were formed beneath the waters of a Paleozoic sea. They occur in well-defined formations, which were folded when the mountains of the region were formed by the compressive stresses of earth movements, and these folds have subsequently been eroded by ordinary atmospheric agencies. These rocks are cut by a few narrow insignificant dikes of igneous rock, which are supposedly connected with the large masses of granite and other igneous rocks now seen at Magnet Cove and Potash Sulphur Springs. In addition to the rocks mentioned there is a considerable area of dark-gray and porous travertine, or calcareous tufa, formed by the Hot Springs.

The sedimentary rocks seen in the vicinity of the Hot Springs consist of shales, sandstones, a few beds of impure limestone, and the rock called novaculite. This last-named rock, of which the well-known Arkansas whetstones are made, is the most conspicuous and important rock in the locality. It is the typical rock of central Arkansas, and, though found over a large area, the material pure enough to be used for whetstones is confined to the vicinity of the Hot Springs. It is this rock that has, by reason of its hardness and its resistance to erosion, made the mountains about the springs, and it forms the cliffs and prominent ledges seen in the district. The bedded rocks form a series shown in the following table, in which the youngest beds are placed at the top of the column and the oldest strata at the bottom.

THE ROCK STRUCTURE.

Near the Hot Springs these rocks have been compressed into great folds which now form the mountains, and this compression is so great that the folds have been pushed over, or overturned, and in the gorge of Hot Springs Creek the section now exposed shows the younger beds resting beneath the older ones. In addition to this there has been some faulting in Indian Mountain, by which an overthrust has pushed up the older beds over younger ones. For this reason the section, as given above, is not always easily made out, but it can be seen in the slopes of West Mountain, although, as will be noted there, the younger beds lie below the older and the rocks have a dip of from 25° to 70°. The Carboniferous shales, which are the youngest rocks of the district, are well exposed on Malvern Avenue near the Park Hotel, where the olive-colored, sandy shales have been found to contain plant stems and fragments of fern fronds. The shales are rarely indurated enough to form slates, though a few quarries have been opened in them and slate of a poor quality extracted. Where the shales are slightly altered they are sometimes valuable for brick and terra-cotta burning, though most of the clay used for that purpose is derived from the disintegrated material washed into the creek bottoms.

The sandstones are of variable texture and composition. The coarser-grained rocks are nearly pure quartzose sand but the intermediate beds are quite clayey. The chief sandstone horizon seen at the springs is the one lying just above the novaculites, and this rock is the one which is so prominent on Hot Springs Mountain and West Mountain.

The novaculites are the most interesting rocks of the region. They consist of nearly pure silica, containing less than one-half of l per cent of other material. The rock is very dense, homogeneous, of a cream or white color, and fine grained, resembling in appearance the finest Carrara marble. These rocks are used for whetstones, the finer grained form being called Arkansas stone and the coarser-grained rock the Ouachita stone. This material has a marked conchoidal fracture and resembles chert in its general appearance, although, as will be shown later, this appearance is purely a superficial one and the material differs markedly from chert in its origin and composition. Although brittle and lacking the toughness of chert, it was extensively used by the Indians, who quarried it by building fires upon the outcrops until the stones were heated and then quenching the fire with water, thus chilling the rock and causing it to split and spall into fragments which were easily removed. In this condition it was readily chipped by the use of round stone hammers, great quantities of which have been found by the early settlers and which the writer has seen at some of the more remote quarries. The rock is finely jointed, and in quarry faces this jointing is more conspicuous than the bedding planes. These phenomena may be well observed in almost any of the excavations seen along the main street above the Government reservation. The finer-grained material seldom forms good outcrops because of this jointing and also because the rock contains a small amount of water, which, when frozen during the frosts of winter, shatters the stone and covers the outcrop with fine débris. This débris is extensively used as a road material, and wherever applied forms a most excellent surface.

The novaculite formation is from 500 to 600 feet in thickness, which includes some flinty shales, some soft shales, and some sandstones. The novaculites proper are prominent members of this formation and occur in beds a few inches to 12 or 15 feet thick. When these beds are less than 4 inches thick the rocks lose the novaculite character, and are more like flinty shales. When examined under the microscope the rock is found to present a very uniform appearance, and to consist of extremely minute interlocking grains of cryptocrystalline silica. Chemical tests show that this silica is quartz and not amorphous silica. Thin sections also disclose the presence of numerous cavities in the rock quarried for whetstones. These cavities have been found to present a rhomboidal outline, and they correspond in form and position to included patches of calcite found in the same rock where the bed passes beneath the creek levels. It has been assumed that these cavities are formed by the dissolution and removal of the calcite, and as the material from beneath the water level is of slight value as a whetstone it has been reasoned that the abrasive qualities of the Arkansas stone are due to the presence of these calcite cavities. The origin of the rock has been the subject of considerable speculation from the earliest times to the present. It has been commonly asserted that it is a very fine-grained sandstone which has been indurated and altered by hot-spring action. This explanation is not adequate, however, since the same beds are exposed on the flanks of the Ouachita Mountain system for a total length of several hundred miles. Moreover, the character of the grains does not permit of the assumption that they were originally rounded and that the spaces between have been filled by a secondary deposition of silica, as is commonly the case with many quartzites. The writer's belief is that the evidence supports the opinion that the rocks were formed as a chemical precipitate in the deep seas of a Silurian ocean, and that comparatively little alteration beyond induration has taken place. Such a theory seems to accord very well with the chemical and physical nature of the rock and with the facts now known in regard to the origin of some of the early geological sediments.

IGNEOUS ROCKS.

Besides the sedimentary rocks just noted there are four narrow dikes of igneous rock about one-half mile south of the mountain borders and near the city limits. These rocks are dark-colored mica traps, a form of rock called "ouchatite." They are chiefly interesting because they show that there was some deep-seated body of molten material from which the dike fissures were supplied. Small dikes are found north of the city, east of the city, and in considerable abundance about Potash Sulphur Springs and at Magnet Cove. These dikes have a generally ESE.—WNW. direction, showing that the fissures are parallel to the mountain sides. They are from l to 4 feet wide and are generally much altered, so that the outcrop is inconspicuous, or is covered by vegetation, and when the rock is broken black mica in small flakes is the only mineral seen.

FOSSILS.

The age of the sedimentary rocks is determined by the fossil remains found in them. The black shales which underlie the novaculites contain remains of a curious hydrozoa. These fossil remains are known as graptolites, and the forms identified at the Hot Springs belong to the upper part of the Lower Silurian age (Trenton and Utica). New types of these fossils peculiar to the Hot Springs are illustrated in the Novaculite report issued by the Arkansas geological survey. Besides these curious forms, a few shell remains (brachiopods and lamellibranchs), corals, and worm trails have been found. The graptolites occur on the north side of the hill on a small stream drainage on the west side of the continuation of Park Avenue. They are also seen in a very black shale forming the bluff on the west side of Park Avenue above the Hotel Hay and below the Barnes House. Similar fossils also occur on Whitington Avenue, one-fourth of a mile above the head of Central Avenue, at a point where the creek crosses the street.

Plant remains of Lower Carboniferous age have been found in the shales exposed in the excavation for a cellar on the western side of Malvern Avenue, 100 feet north of the Park Hotel. The shales are varicolored, brown, red, gray, and black, but the fossils occur in the olive-colored, sandy shales. Similar fossils were also found in Ouachita Avenue at the Hot Springs.