GEOLOGICAL SURVEY OF THE TERRITORIES. 35 



west, inclining north 75°. The rock is rusty, reddish quartzite, or an 

 aggregate of particles of quartz. The Silurian and carboniferous rocks 

 appear here and there, but the tertiary beds are the most conspicuous. 

 Near St. Mary's Station the beds of conglomerate project out of the 

 bluffs, on both sides of the Sweetwater, quite consxhcuously. On the 

 distant hills the arenaceous marls are much exposed in the form of naked 

 hills, at the base of which Mr. Elliot discovered some very interesting 

 turtle remains. About a mile above St. Mary's Station the Sweetwater 

 flows out of a deep gorge or canon, cutting the ridges of older rocks at 

 right angles. The strike is northwest and southeast, dip northeast. 



The limestones and sandstones are very conspicuous. On the north 

 side we pass over the upturned edges of an enormous thickness for three • 

 fourths of a mile, inclining 30° to 40°. These ridges form the east side 

 of the anticlinal that runs down from the Wind Eiver Mountains. 

 Indeed it is a portion of the range itself. So far as the east side is con- 

 cerned I have never seen a more perfect anticlinal. The series of ridges 

 of carboniferous limestones and Potsdam sandstones extends across the 

 country in regular lines, and the erosion has been such that the out- 

 cropping edges are but little above the general surface, so that they can 

 be studied, with ease. The main road passes across the edges of all the 

 rocks at right angles. Not far distant to the northwest the snowy peaks 

 of the Wind Eiver range rise high above the surrounding country. 

 Here and there, resting upon the edges of the older rocks, are patches of 

 the modern tertiary deposits, remnants of the last period of erosion. 

 As we approach the base of the mountains there is a sJght dip in these 

 tertiary beds 3° to 5°, as if the last movements were subsequent to their 

 deposition. There is a deep valley from the base of the mountains to 

 the Sweetwater, which marks the line of separation between the Silurian 

 and carboniferous ridges, completely separates them, but indicates no 

 discordancy. Inside of these ridges is an immense thickness of slates 

 standing nearly vertical, with a strike northeast and southwest, inclining 

 slightly northwest. Extending to the northeast toward the Wind Eiver 

 Valley could be seen ridge after ridge of cretaceous and tertiary beds. 



CHAPTEE III. 



FROM SOUTH PASS TO FORT BRIDGER. 



Like the Black Hills of Dakota and the Laramie range, the Wind 

 Eiver Mountains form a complete anticlinal. It is so regular that when 

 once the key to its structure is obtained it is studied with great ease. 

 So far as my observations have extended, all the ranges with a north- 

 west and southeast axis are simple and regular in their structure. They 

 may be defined briefly as a nucleus of granite or gneissic rocks, rising 

 step by step on either side toward a central axis, and on each side of 

 the nucleus the various unchanged rocks inclining at a variety of angles. 

 Sometimes extensive erosion and the subsequent deposition of modern 

 tertiary beds or drift obscures the study, and we shall find that this is 

 the case with the Wind Eiver range. The Black Hiils of Dakota is the 

 most complete illustration of an anticlinal, not complicated by other 

 influences, that I have met with in the West. The nucleus is massive 

 feldspathic granite, with a series of gneissic beds outside of it ,• these 

 incline in every direction from this nucleus, a sort of narrow, ova) 

 quaquaversal, and all the unchanged beds known in this pwtion of the 



