THE PORCUPINE MOUNTAINS, 209 
are first found to the east on the Ontonagon River, and thence extend west- 
ward all the way to and beyond Bad River, in Wisconsin. Irom this cen- 
tral mass of porphyry the remaining beds of the Lower Division, and those 
of the Upper, dip away on all sides in a manner that will be best under- 
stood from a study of the map and sections of Plates XIX, XX and XXI. 
Above the porphyry comes first a succession of beds of diabase, mela- 
phyr, amygdaloid and diabase-porphyry, with one included bed of porphyry- 
conglomerate, in all 400 feet in thickness. To this sueceed 1,900 feet of 
sandstone and conglomerate; 300 to 400 feet of beds of diabase and diabase- 
amygdaloid; upwards of 8,000 feet of sandstone and conglomerate ; 600 
feet of dark sandstone and black shale; and then the main mass of sandstone 
of the Upper Division. ‘This succession is plainly enough the same as that 
which obtains on the Ontonagon, while the black slate, outer conglomerate, 
trap, and lower conglomerate, are as plainly the equivalents of the shales, 
Outer Conglomerate, Lake-Shore Trap, and main or Great Conglomerate of 
Keweenaw Point. 
The structure of the Porcupine Mountains having never before been 
worked out, nor indeed any of the constituent rock beds deseribed, save in 
the roughest way, I may properly include here somewhat detailed de- 
scriptions of each of the members of the suecession above laid down, Plates 
XIX, XX and XXI should be studied in connection with these descriptions. 
The porphyry which constitutes the central mass of the mountains ap- 
pears in numerous bold, structureless exposures through the middle, from 
south to north, of T. 50, R. 44 W.; in the northern parts of the north- 
western sections of 'T. 50, R. 43 W.; in the southwestern sections of Il’. 51, 
R. 42 W., especially on the upper Carp River; and over a large area in 
the southern and southwestern sections of 'T. 51, R. 43 W. The ridges in 
this area rise often 1,000 to 1,200 feet above Lake Superior, while the lower 
portions rarely sink to 800 feet above the same level. 
The rock seen on the various exposures within this area varies in color 
through various shades of red, lilac and purple. It shows everywhere a 
distinct tendency towards becoming a non-porphyritie felsite, porphyritic 
quartz and feldspar being sometimes altogether absent, though more com- 
monly present in small and sparsely scattered particles. There is ocea- 
144L8 
