CHARACTERS OF THE KEWEENAWAN ROCKS. 29 
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46 per cent. silica, to the most acid of the acid kinds subsequently men- 
tioned. 
Interstratified with these basic crystalline rocks, at many different 
horizons, but generally greatly more abundant above, are detrital beds, 
chiefly reddish conglomerate and sandstone. The conglomerates are for the 
most part made up of pebbles of one or more of three kinds of acid rocks, 
viz: (1) a red to brown or purple felsite, nearly or quite without either 
quartz or orthoclase as porphyritic ingredients; (2) a true quartziferous 
porphyry, usually brick-red in color; (3) a non-quartziferous porphyry, 
with bright-red striated feldspar crystals; and (4) fine-grained to coarse- 
grained granitic porphyry and augite-syenite, of which the several phases 
verge towards quartziferous porphyry or granite on the one hand, and 
quartzless porphyry or an orthoclase-bearing gabbro on the other. The 
first two kinds are commonly without structural lines, but occasionally show 
faint and wavy bandings. One or two of these kinds will be found to pre- 
dominate greatly at any one place, the same conglomerate belt showing at 
different points along its course great differences in this respect. 
Pebbles of the basic rocks also occur in the porphyry- conglomerates, 
but they are relatively very rare. Pebbles of altogether different rocks are 
occasionally seen; as, for instance, on the peninsula of Mamainse, on the 
east coast, where granitic and gneissic pebbles are abundant. The matrix 
of these conglomerates appears to be of the same material as the pebbles 
themselves, and in the coarser kinds can easily be seen to be so. It is fre- 
quently permeated by calcite, which at times has completely replaced the 
matrix, yielding a striking looking combination of red and brown pebbles 
with a background of pure white cleavable calcite. 
An altogether different conglomerate from those just described, and one 
of much more restricted distribution, has a red shaly matrix, often finely 
laminated, in which the pebbles are wholly of the common diabase and dia- 
base-amygdaloid. Of these conglomerates there seem to be again two 
kinds; one in which the pebbles are distinetly waterworn, and another in 
which there is no such distinet evidence of water action, and in which the 
vesicular exteriors of the balls suggest their possible origin as volcanic scorize 
that have become buried in the accumulating detritus. The first of these 
