Geological Survey Bulletin 1309 The Geologic Story of Isle Royale National Park

PREHNITE—THE LITTLE PINK PEBBLES

Prehnite is an abundant secondary mineral in some lava flows on Isle Royale and elsewhere in the Lake Superior region. It occurs as amygdule fillings, crosscutting veins, and as a replacement of earlier minerals or rock. Most of the prehnite has a characteristic pale-green to white color, but that in amygdules is commonly light to dark pink or variously mottled in pink and green. The pink prehnite superficially resembles thomsonite, with which it has commonly been confused; most of the so-called thomsonite from Isle Royale is actually pink prehnite, including material from Thomsonite Beach on the north side of the island. The prehnite does not develop the spectacular patterns and color variations present in gem-quality thomsonite, which explains why "Isle Royale thomsonite" has always been considered to be of inferior quality. Nevertheless, the prehnite amygdules polish nicely and are attractive in themselves.

The pink prehnite amygdules, which most commonly range in size from 1/2 to 1 centimeter, are more resistant to erosion than the volcanic rock matrix within which they have formed. As a result, beach pebbles containing them often have a knobby appearance, the amygdules projecting above the general surface of the matrix (fig. 73). Where the prehnite amygdules weather completely free from the matrix, they may make up a fair percentage of the fine gravel on beaches near the prehnite-bearing outcrops.

PREHNITE AMYGDULES in beach pebble derived from lava flow. Long dimension of pebble is 5 centimetres. (Fig. 73)

The amygdules in figure 74 are typical and illustrate the radiating fibrous habit, with the occasional development of "eyes," that has probably been a factor in their confusion with thomsonite. The pink color of the prehnite is due to internal reflections from finely disseminated native copper inclusions, and the color intensity is related to the distribution, abundance, and grain size of the inclusions.

PREHNITE AMYGDULES—note radiating fibrous structure with development of "eyes." (Fig. 74)

AGATE—AN ARRAY OF COLORS

Perhaps the best known gemstones of the Lake Superior region are agates. Although they are most plentiful near where they weather from the enclosing rocks, glacial transport has caused them to be widely distributed, and there are few pebble beaches where one cannot be found.

Agate is a subvariety of chalcedony (fibrous quartz) with a distinct banding in which successive layers differ in color and in degree of translucency. It most commonly originates as a cavity filling in volcanic rocks either as amygdules in volcanic flows or as irregular-shaped masses in volcanic tuffs; both types occur in the Lake Superior region, including Isle Royale.

One of the most striking examples of an agate-bearing lava flow in the park is the Amygdaloid Island Flow. This flow, exposed only on Amygdaloid Island, has rather abundant rounded or almond-shaped agates with a characteristic flesh-pink color and commonly massive quartz or vuggy centers (fig. 75). Agates from an individual flow tend to be somewhat similar in appearance; for example, those from the Amygdaloid Island Flow are pinkish, whereas those from the Long Island Flow tend to be bluish. In general, agates from the rocks on Isle Royale are paler and have less color contrast between color bands than those found in some other parts of the Lake Superior region.

AGATE AMYGDULES in fine-grained trap from Amygdaloid Island Flow. (Fig. 75)

The volcanic tuff that overlies the Greenstone Flow also contains numerous agates with a pink or red cast (fig. 76). Unlike the ovoid agates typically occurring in the lava flows, these agates tend to have very irregular shapes similar to the so-called thunder-egg agates occurring in welded tuffs of the Columbia River Plateau in the northwestern United States, and they probably had a similar origin. The welded tuffs formed from volcanic ash that fell in a hot plastic condition that permitted it to be fused or welded into a generally cohesive mass. The agates themselves are interpreted as chalcedony deposited in cavities formed by pockets of gases accumulating during cooling, shrinkage, and partial crystallization of the tuff.

AGATE typical of that in the volcanic tuff above the Greenstone Flow. (Fig. 76)

Because of their hardness, agates are more resistant to erosion than the enclosing rock and tend to become concentrated in gravel and other surficial deposits both ancient and modern. Thus agates are present in the conglomerates interbedded with the lava flows on the island and in the Copper Harbor Conglomerate, as well as in the modern surficial deposits. Most of the agates on beaches near conglomerate outcrops have, therefore, gone through two cycles of weathering and release from their host rock—first from the volcanic rock in which they originated and then from the conglomerate in which they were subsequently incorporated. In addition to beach agates derived from nearby bedrock, some have been winnowed from glacial till by wave action and may have been transported a long distance from their original source; they commonly are quite different in some way from the majority of the agates on a given beach.