72 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



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stone an inch or two in thickness, composed of trilobites mostly in 



a fragmentary condition, but with 

 now and then a specimen so pre- 

 served as to characterize it. There 

 were great numbers and variety of 

 these okl Sihirian fossils, and they 

 undoubtedly belong to the Potsdam 

 group. The sections represented by 

 Figs. 20, 21, and 22 are continuous and 

 occupy a distance of about thirty miles. 

 The accompanying illustration will 

 show the consecutive series of strata 

 with their inclination. (Fig. 22.) The 

 thickness of the entire series of 

 Lower Silurian strata here is esti- 

 mated at 1,0U0 feet. The massive lime- 

 stones which I have referred to the 

 Potsdam group are about 400 feet 

 thick, then gradually pass down into 

 50 feet of thinly-laminated, cherty 

 limestone or calcareous mud-layers, 

 with abundant organic remains. Then 

 come layers of greenish sand and 

 clays with shells and trilobites ciuite 

 distinct from those above. Some of 

 the layers of sandstone have small 

 rounded pebbles, though . not prop- 

 erly a conglomerate. Then comes a 

 puri)lish sandstone, and below these 

 variegated shaly clays, yellow, green, 

 &c., theu hard, dull purplish-brown 

 quartzose sandstone, inclining 45°, 

 ai)parently metamorphosed in part, 

 80 feet. Then comes a grassy inter- 

 val, and then a ridge of very compact 

 brownish-gray quartzite, with irreg- 

 ular layers of sandstone, inclining 

 45°. Then alternate layers of brown 

 calcareous sandstone and yellowish- 

 brown shaly clays. Then drab-brown 

 quartzites and black slates alternate, 

 150 feet. Then comes the steel-brown 

 quartzite, which has the appearance at 

 a little distance in the ft-acture of com- 

 pact basalt, 100 feet. Then a series of 

 black slates 150 feet thick. Then 

 comes a dark micaceous sandstone, 

 alternating with calcareous slates or 

 shales, 300 to 500 feet in thickness. 

 This last group of beds continues 

 along the river in high bluffs for 

 about live miles and presents a great 

 variety of structure. In some in- 

 stances the sliales weather to a soft, 

 yellow, chalky material, and remind 

 one at a distance of the yellow-chalk 

 Then Ave have a brown somewhat worn 



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beds of the Middle Cretaceous. 



