162 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



Figure 15 will serve to show the appearance of the ridges of upheaval, 

 or " hog backs," as they are sometimes called in the west. These ridges 

 occur to a greater or less extent all along the eastern flanks of the Boeky 

 Mountains. They sometimes rise like steps to the crest, so that one 

 may walk from the tertiary formations in the plains, holding a horizon- 

 tal position, across the uplifted edges of all the formations to the granite 

 nucleus. Between these ridges are, in many instances, beautiful grassy 

 valleys, varying from a few hundred yards to half a mile in width. 



When exposed only on one side of a range I have called them mono- 

 clinal ridges and valleys. 



Fig. 15. 



Ridges of uplieavel near Big Thompson Creek, Colorado. 



The railroad then for forty miles passes over and cuts through a great 

 variety of syenites; some compact, beautiful building stones, almost 

 equal to the Scotch syenites, but the greater part ferruginous and easily 

 disintegrating on exposure. ,•** 



On the west side of the Laramie range we pass across the uplitted 

 edges of the various formations seen on the eastern margin. We know 

 that the carboniferous limestones occur here, for the fossils which, 

 though not abundant, are explicit as to their age. The limestones lie 

 all along the margins of the Laramie range up to the Bed Buttes, and 

 sometimes slope so gently and cover the flanks so uniformly as to appear 

 like a sort of flexible roof. Along the North Platte, above Laramie 

 Peak, the little streams have cut deep canons through the limestones 

 and sandstones of carboniferous age. In Box Elder Canon, Be mipronites 

 crassus, Broductus semieticulatus, Aviculapecten occidentalism and other 

 well-known carboniferous forms occur. Between the granites and the 



