STRUCTURAL FEATURES OF THE DETRITAL ROCKS. 151 
ability to a fold, the porphyry of the central portions being one of the 
usual embedded masses laid bare by subsequent denudation. 
The detrital members of the series do not present any structural 
features that are peculiar to them as compared with similar mechanical sed- 
iments of other regions. The conglomerate layers reach sometimes an 
extraordinary thickness, and are often made up so completely of large-sized 
pebbles that it is necessary to believe in the existence in such places of very 
powerful currents. Traced laterally, these conglomerate belts have often 
a great extent, but they never remain constant as to coarseness or exact 
nature of materials. The outer conglomerate of Keneenaw Point for in- 
stance—that seen from Copper Harbor eastward to the extremity of the 
point—is undoubtedly the same as the outer sandstone and conglomerate, 
immediately beneath the black slate of the Porcupine Mountains, and is the 
same as the great conglomerate of the Montreal, Potato and Bad rivers, 
in Wisconsin. It has thus a continuous lateral extent as a single layer of 
at least 170 miles, and possibly a much greater extent; but in this long 
distance it varies from less than 100 to 4,000 feet in thickness, is now pure 
sandstone, now nearly all conglomerate, now a sandstone with conglomerate 
bands, and again a coarse bowlder-conglomerate. Moreover, as already 
noticed, the nature of the pebbles, while always predominatingly of some 
one of the acid rocks of the series, presents many variations along the 
course of the belt, depending upon similar variations in their source of sup- 
ply. The inner broad conglomerate of Keweenaw Point, which is devel- 
oped so largely at the mouth of Eagle River, is also represented in the 
Ontonagon and Porcupine Mountain regions, but on the Montreal has nar- 
rowed down to a mere sandstone seam. 
Of the thinner conglomerates which are intercalated at all horizons in 
the Keweenaw Point region, one, the so-called Albany and Boston, lying 
immediately underneath the Greenstone, has been traced for a distance 
of as much as 50 miles, but varies along its course from a full conglom- 
erate toa mere red shale seam. From what I have seen I have little doubt 
that a number of the thinner conglomerates have even a greater extent 
than this. 
