THE EAGLE RIVER SECTION. 167 
somewhat irregularly, and it results that the thickness above mentioned— 
25,000 feet—is met with in only one section. ; 
How this thickness is built up I shall try to show by first describing 
somewhat fully the section measured so carefully by Marvine in a S. 28° E. 
direction from the mouth of Eagle River to the Phcenix mine. This section 
I shall next extend roughly downwards to the Eastern Sandstone; adding 
then brief descriptions of the still higher and lower layers seen eastward 
from Eagle River, the maximum thickness will be reached. <A brief sum- 
mary of the results of this study will then serve to make them clearer to the 
reader. 
Marvine’s work on the Eagle River section did not include microscopic 
study, but this has since been added by Pumpelly, besides which I have my- 
self examined a number of sections of specimens from here. I have also 
studied Marvine’s descriptions on the ground. 
The northernmost rock of this section, seen in the streets of the town of 
Eagle River and on the banks of the stream near its mouth, is a pebble- 
and bowlder-conglomerate of great thickness. The horizontal width of 
this conglomerate exposed is about 1,300 feet, corresponding with a dip of 
31° to a thickness of some 700 feet. Higher layers are covered by the 
lake waters, the overlying diabases and amygdaloids forming a reef some 
3,400 feet out in the lake from the mouth of Eagle River. The whole 
width of the conglomerate thus appears to be some 4,500 feet, and its thick- 
ness 2,000 to 2,300 feet. 
The always rounded pebbles of this conglomerate, which average from 
3 to 8 inches, sometimes reaching a foot, in diameter, and are coarser in 
some layers than in others, are of four principal kinds. The greatly pre- 
dominating sort is of a chocolate-brown non-quartziferous porphyry, with a 
very hard, aphanitic, readily fusible matrix, scattered through which are nu- 
merous small, pinkish feldspar crystals. Under the microscope the section 
of these pebbles shows a matrix apparently wholly crystalline, all portions 
polarizing in some part of a revolution, and seemingly wholly of orthoclase 
particles. Thickly scattered through the matrix are minute brown, black 
and red particles, which must be chiefly iron oxide. The porphyritie erys- 
tals are mostly orthoclase, but some of the larger ones are oligoclase. 
