DETRITAL ROOKS. 127 
Section IV.—DETRITAL ROCKS. 
The fragmental rocks of the Keweenaw Series include, as already 
shown, both conglomerates and sandstones. The former have already been 
sufficiently described in a general way, the several kinds of bowlders and 
pebbles of which they are made up having been also described in detail in 
the foregoing pages of this chapter in connection with the descriptions of 
the original massive rocks from which they have been derived. Descrip- 
tions of the more important conglomerate beds are also given in connection 
with the local descriptions of Chapters VI and VII. It is therefore unnec- 
essary to give any further account of them here. 
It may merely be said, in review, that the ordinary conglomerates are 
for the most part made up of pebbles and bowlders worn from the several 
kinds of acid rocks, viz: quartzless porphyry, quartziferous porphyry and 
felsite, augite-syenite, granitell and granite; that the same band of con- 
glomerate will vary when followed along its course as to which of these kinds 
is the predominant one; that there are often pebbles of the basic rocks, 
but in greatly subordinated quantity; that the finer material between the 
pebbles is composed of a detritus of the same rocks as the pebbles them- 
selves, the only difference being that the greater fineness of these particles 
has often resulted in separating crystals of feldspar and quartz completely 
from the matrix; that one, or more, of calcite, chlorite, epidote and cop- 
per—brought in by infiltrating waters—has often been deposited in the 
interstices of these conglomerates, or rather has replaced their constituent 
particles; and that these conglomerates tend, even when quite thin, to run 
laterally into sandstone or shale, the coarser fragments failing altogether. 
The sandstones seem in large measure to be made up of particles of 
the same acid rocks whose fragments form the conglomerates. When they 
closely overlie beds of basic rocks, these sandstones often contain a greater 
or less proportion of basic detritus. This basic material presents itself in 
the shape of pieces of more or less altered amygdaloid matrix, particles of 
the more completely crystalline rocks with the constituent minerals still 
adhering to one another, and pieces of these minerals broken away from 
