70 GEOLOGICAL SURVEY OF THE TEREITORIES. 



toid rocks at a greater or less angle. There seems to be no unconform- 

 ability in these ibssiliferous rocks from the Potsdam inclusive to the 

 top of the Cretaceous. 



From these facts we draw the inference that prior to the elevation of 

 the Black Hills, which must have occurred after the deposition of the 

 Cretaceous rocks, all these formations presented an unbroken continuity 

 over the whole area occupied by these mountains. This is an impor- 

 tant conclusion, and we shall hereafter see its application to other 

 ranges, and also to the Eocky Mountain range taken in the aggre- 

 gate. 



Proceeding in a southwest direction from the Black Hills, we find 

 that there are ample proofs of the connection of these hills with the 

 Laramie Mountains, through a low anticlinal which can be followed for 

 many miles. It is sometimes concealed by the recent Tertiary beds, but 

 it re-appears at different points. By the Laramie Mountains we desig- 

 nate those eastern ranges which extend from the Eed Buttes southward 

 to the Arkansas. This range, when examined in detail, is composed of 

 a large number of smaller ranges, all, so far as I have observed, of the 

 true granitic type. The trend of the whole group is very nearly north 

 and south, northward as far as Fort Laramie, where they make an ab- 

 rupt flexure around to the west and northwest, and gradually cease and 

 die out at the Eed Buttes. From this point westward and northward, 

 there is a space from 25 to 40 miles in width, destitute of mountain-ele- 

 vations, though the strata exhibit evidence of dislocation or crust move- 

 ments. 



Geologically the Laramie range is also composed of a granitoid nu- 

 cleus, with the fossiliferous formations, Silurian, Carboniferous, Eed Are- 

 naceous beds, (Triassic,) Jurassic, Cretaceous, and in many places Lignite 

 Tertiary, inclining from each side of a central axis at various angles. It 

 is in these mountains that the numerous branches of the Platte have 

 their sources, extending a distance of nearly 400 miles. From the ob- 

 servations which I have made in this range, it seems to me that the 

 conclusion is plain that all the above-named rocks in a nearly or quite 

 horizontal position were continuous over the whole area at present oc- 

 cupied by it some time during the Tertiary j^eriod. 



The most important outlier of the Eocky Mountains on the eastern 

 slope is the Big Horn range, which, though somewhat irregular in the 

 shape of its mass, has a general trend nearly northwest and southeast. 

 It occupies an area about 180 miles in length and 50 in breadth, near 

 latitude 43° 30', and longitude 102°. The line of fracture seems to have 

 partially died out as it were toward the south or southeast, and to have 

 made a general flexure around to the west, the whole range soon losing 

 its granitoid character and becoming entirely composed of more modern 

 eruptive rocks. The eruptive portion continues westward until it joins 

 on to the Wind Eiver range, near the sources of Wind Eiver, at the 

 southern end of the Big Horn Mountains. We can trace a single anti- 

 clinal across the prairie connecting these mountains with the Laramie 

 range at the Eed Buttes on the North Platte. We also know by the 

 position of the sedimentary beds upheaved along the mountains that 

 these mountains also form a connection with the Wind Eiver range by 

 the gradual flexure westward of the eruptive rocks. The central por- 

 tion of these mountains is also composed of granite and granitoid 

 rocks, with the same series of fossiliferous formations, inclining at vari- 

 ous angles from each side of the axis of elevation, as are seen around 

 the Black Hills an-d along the Laramie Mountains. Some of the more 

 lofty peaks are from 8,000 to 12,000 feet above the sea, and are covered 



