82 GEOLOGICAL SURVEY OF THE TEREITORIES. 



by two separate minor ranges projecting out from the main range of 

 mountains, and the trend of these minor ranges is nearly north and 

 south. One of the small ranges is quite peculiar in its character. On 

 its east base, which fronts on Laramie Plains, the Upper Cretaceous beds 

 jut up against its side, and no unchanged rocks of older date are seen, 

 while on the west side, about five miles distant in a straight line, the 

 entire series, from the Carboniferous to the summit of No. 3 Cretaceous, 

 are all visible, inclining at greater or less angles from the slope. 



The nucleus of the mountain is syenite, of various degrees of fineness 

 and compactness, inclining at a large angle, from 50° to 70°, toward the 

 southeast, or nearly east. It is an important question to determine the 

 exact relation of these metamorphic rocks, which form the central por- 

 tion of all the mountain-ranges, to the, unchanged oeds which usually 

 incline from their sides. Do they conform to each other or not ? Did 

 the metamorphic rocks lie in a more or less inclined position prior to 

 the deposition of the Silurian or Carboniferous beds upon them '? 



We have thus found it difficult to determine the conformability or 

 unconformability west of the Laramie range, but on the east side of the 

 mountains, especially near Fort Laramie, and along the eastern slope of 

 the Big Horn and Wind Eiver Mountains, the discordant relation of the 

 two series is very apparent. 



These question will have a most important bearing when we attempt 

 to reconstruct the history of the physical revolutions which have oc- 

 curred in the West during past geological epochs. 



The syenite beds which form the nucleus of the small range of mount- 

 ains between the Big and Little Laramie Eivers, inclining eastward, 

 were pushed up in such a way that the east front is almost vertical, and 

 the Cretaceous beds at the foot, which must have been borne upward in 

 part during the elevation, have fallen abruptly down, so that in some 

 instances they have passed the vertical position 20° to 30°. 



East of the Big Laramie, and all along the western slope of the Lara- 

 mie range, the entire series of unchanged rocks are visible, inclining at 

 moderate angles from the mountain-sides. On the west side of this 

 range the slope is more gentle, and the Carboniferous, Triassic, Jurassic, 

 and Cretaceous beds present their upturned edges clearly to the scrutiny 

 of the geologist. 



The synclinal valley here, through which the Little Laramie flows, is 

 about five miles wide, and crossing this stream west, we find the full 

 series inclining from the mountain eastward. The dip of the red beds 

 is from 40° to 60°, that of the Cretaceous 40°. No fossils have been 

 found in any of the unchanged rocks below No. 3 Cretaceous, west of 

 Fort Sanders, nor does the nature of the beds indicate that the physi- 

 cal conditions during their deposition were favorable for the existence 

 of animal or vegetable life, certainly not for the preservation of its re- 

 mains. 



Between the well-marked Cretaceous beds and the metamorphic rocks 

 nearly all the rocks are of a brick-red color, or tinged more or less with 

 red from the presence of the peroxide of iron, and diffused through them 

 there is a certain amount of gypsum ; hence they have been called gyp- 

 siferous deposits. In the Black Hills, Big Horn, and Wind River Mount- 

 ains, these red beds are largely developed, and there they contain beds 

 of beautiful white amorphous gypsum, varying in thickness from 5 to 

 60 feet. Along the east slope, near Pike's Peak in Colorado, these for- 

 mations contain valuable beds of gypsum, but in the Laramie Plains I 

 have as vet observed no regular beds. The thickness of these deposits 



