12 COPPER-BHARING ROCKS OF LAKE SUPERIOR. 
seem to separate or open out to inclose the imbedded balls. In appearance it is un- 
distinguishable from the finer-grained sandstones, though perhaps containing more 
calcite, which is more often collected into small generally lenticular cavities, which 
are sometimes more numerous in rude narrow bands, parallel with the bedding, than 
is the case with the sandstones.’ 
That one of these beds which lies at the top of the group has a total 
thickness of 86 feet, of which 12 are the scoriaceous amygdaloid, 6 pseud- 
amygdaloid, and 68 compact portion. The corresponding figures for the 
basal bed of the group are 7 feet, 7 feet, and 31 feet—total, 45 feet. The 
porphyritic diabase characterizing the lower compact portions of these two 
beds is the type of the kind described in Chapter III, under Pumpelly’s 
name of ashbed-diabase. This diabase is remarkable for its small amount 
of augite, the plagioclases making up most of the thin section, and also for 
its not having the augite contours determined by the outlines of the older — 
feldspar crystals. 
Between these two limits the Ashbed Group displays a considerable 
variation in its subordinate beds. Immediately below the uppermost bed 
just described (Marvine’s 45) come ten thin layers, for the most part under 
15 feet in thickness, with very strongly marked and thin amygdaloids and 
pseud-amygdaloids. The amygdules of the amygdaloids are chiefly cal- 
cite and prehnite, less commonly laumontite, while in the pseud-amygda- 
loids the pseud-amygdules are chiefly a greenish, soft, chloritic substance. 
Below these again comes a thickness of ‘curious, coarse, easily decompos- 
able beds of irregular structure,” of a dull reddish-brown color, mottled with 
dark green, or vice-versa, one bed being characterized by the very unusual . 
production, on a large scale, of analcite as a pseud-amygdule. Beneath 
these layers again is first a bed of semi-columnar, hard, dark-greenish, very 
fine-grained rock, allied to the ashbed type of rocks. Then follows a thin 
seam of sandstone a few inches in thickness. Below the sandstone are two 
heavy beds (64 and 65) of the ashbed type of diabase, with strongly marked 
amyegdaloids. 
The massive portion of bed 64, according to Pumpelly, is a 
dark-green, almost black, erypto-crystalline rock, which is easily scratched with the 
knife. Under the microscope, it is found to consist chiefly of plagioclase in very small 
1Geol. Survey of Mich., Vol. I, Part II, p. 125. 
