26 GEOLOGICAL SUKVEY OF THE TEKRITOEIES. 



times becoming a sort of slaty shale. South of the railroad it contains 

 very few fossils. It is not a thick bed, rarely exceeding 100 feet, and 

 gradually passes up into the yellow arenaceous clays of No. 5. No. 4 

 usually underlies one of the parallel valleys, aud can only be studied to 

 advantage when a stream ilowiug down from the mountains into the 

 plains cuts a channel through it. In these localities the gradual litho- 

 logical changes may be observed. A low ridge is formed by the beds of 

 transition between Nos. 4 aud 5, which is sometimes so regular and con- 

 tinuous that it looks like the rnius of an old wall. The shale gradually 

 becomes dull brown, indurated clays alternating with thin layers of mud- 

 sandstone, aud finally rising up to rather thick sandstones of various 

 degrees of brown and rusty-yellow color. In some localities the fossils 

 are quite abundant, as in the valley of Cache a la Poudre, near Greeley, 

 and in the valley of Big Thompson, west of the Denver Pacific Eailroad. 

 Here are found rounded concretiouary masses, like huge cannon-balls, 

 resembling those of the same formation on the Cannon-ball Eiver, 

 which empties into the Missouri Eiver from the East, far up in Dakota 

 Territory. These singular concretions contain quantities of trell-marked 

 Cretaceous fossils. Ammonites, BacuUtes, Inocermmts, &c., &c. This 

 group of calcareous sandstones, which reaches a thickness of 100 to 200 

 feet in this region, I have regarded as the upper portion of the true 

 Cretaceous groups. These pass gradually up into the Lignitic formation, 

 about the age of which there is some discussion among paleontologists. 

 North of Saint Vrain's Fork, the Lignitic group, as well as the upper por- 

 tion of the Cretaceous, is seldom lifted up at a high angle, usually 3^ to 

 15°, the angle t»f inclination becoming less and less as the ridges die out 

 eastward in the plains. The lower portion of the Lignitic group shows 

 the influence of the forces that elevated the mountains, but soon they 

 become nearly or quite horizontal, aud far east of the Cache a la Poudre 

 pass under the White Eiver deposits. The latter thin out in their south- 

 •ern extension, so that they are not unfrequently worn down to the un- 

 derlying Lignitic beds. About twenty miles south of Cheyenne there is 

 a bed of coal 5 to 6 feet in thickness, and above it is a bed of oyster- 

 shells 4 feet in thickness. Many of them are quite perfect, but they are 

 mostly fragmentary and worn, as if they had drifted into this locality. 

 In the lower portion of the Lignitic group it is very common to meet with 

 seams or beds of Ostrea, of various species. This species resembles 

 very closely a form holding about the same position on the Upper Mis- 

 souri, in the valley of Grand Eiver, known as Ostrea subtrigonalis. In 

 both localities it occurs in the lower portion of the Lignitic group, and 

 in the ascent gives place to purely fresh- water types. At the Mar- 

 shall coal-mines, south of Boulder City, I have collected small forms of 

 Ostrea^ in strata above the most important beds of coal, and also along the 

 Un on Pacific Eailroad, at Point of Eocks, Black Buttes, and other sta- 

 tions, where the coal-beds are very numerous and their order of succes- 

 sion well shown. The various forms of the genus Ostrea are abundant, 

 passing up through several hundred feet of the coal group. 



It is not my purpose to enter into a discussion of the age of these beds 

 in this connection. In a previous report I stated that the relations of 

 the well-defined Cretaceous group with the Lignitic forms one of the 

 most important problems in Western geology, and that no effort would 

 be spared to accumulate all the evidence bearing on the question possi- 

 ble. I believe that the area for the solution of the question lies in the 

 Laramie plains, and westward toward Salt Lake. There in the aggre- 

 gate are 10,000 to 12,000 feet of Cretaceous and Lignitic strata. Contigu- 

 ous districts may aid in throwing light on the subject, but with all that 



