72 



GEOLOGICAL SUEVEY OF THE TERRITORIES. 



stone an inch or two in tbickness, 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 Silurian fossils, and they 

 undoubtedly belong to the Potsdam 

 group. The sections reprcvsented 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 



1 Lower Silurian strata here is esti- 

 I mated at 1,600 feet. The massive iime- 

 p- stones which I have referred to the 

 .. Potsdam group are about 400 feet 

 I thick, then gradually pass down into 

 g 50 feet of thinly-laminated, cherfcy' 

 ^^ limestone or calcareous mud-layers, 



p ^ with abundant organic remains. Then 

 J -§ come layers of . greenish sand and 

 g I clays with shells and trilobites quite 

 g -§ distinct from those above. Some of 

 ^ I the layers of sandstone have small 

 a "^ rounded pebbles, though not prop- 

 I I erly a conglomerate. Then comes a 

 ^ a purplish sandstone, and below these 

 g £ variegated shaly clays, yellow, green, 

 f. i &c., then hard, dull purplish-brown 

 a I quartzose sandstone, inclining 45°, 

 I I* apparently metamorphosed in part, 

 B J 80 feet. Then comes a grassy iiiter- 

 I o val, and then a ridge of very compact 

 8 pq" brownish-gray quartzite, with irreg- 

 - ular layers of sandstone, inclining 

 J 45°. Then alternate layers of brown 

 S calcareous sandstone and yellowish- 

 3 brown shaly clays. Then drab-brown 

 I quartzites and black slates alternate, 

 ^ 150 feet. Then comes the steel-brown 

 g quartzite, which has the appearance at 

 I a little distance in the fracture of co^^n- 

 ^. 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 five miles and i)reseuts a great 

 variety of structure. In some in- 

 stances the shales weather to a soft, 

 yellow, chalky material, and remind ■ 

 one at a distance of the yellow-chalk 

 beds of the Middle Cretaceous. Then we have a brown somewhat worn 



