76 GEOLOGICAL SURVEY OF THE TERRITORIES. 



is a considerable thickness of what may properly be called transition beds, 

 or beds of passage, which will probably remain a long time in doubt. 



From Pine Grove we continue on our course to the eastward, with the 

 high wall of sandstone on the left and the plateau on the right. Nearly 

 opposite Pine Grove Station the anticlinal that extends off toward 

 Bawlings' Springs forms a notch or triangular area, in which the cre- 

 taceous clays are worn into three singular, terrace-like ridges on the 

 west side, with a strike southwest and northeast, and on the east side, 

 northwest and southeast. 



The valley of Sage Creek is three to six miles wide. On the North 

 Platte there is a fine exhibition of the sandstones or transition beds. 

 Near the crossing of the old stage road there are vertical bluffs eighty 

 to one hundred feet, composed of grayish-brown sandstone, which exhib- 

 its, in the most remarkable degree, the various signs of shallow- water 

 depositions, as ripple, rain, and mud-markings, with what appear to be 

 trails of worms, &c. Broad, flat masses of sandstone lie at the base of 

 the bluff fifteen or twenty feet square, with the surface covered with 

 these peculiar markings; oblique layers are not uncommon. The in- 

 durated clays of well-known cretaceous origin are well shown here, ex- 

 tending up the North Platte for fifteen or twenty miles to the south- 

 ward, while, to the north, ridge after ridge extends as far as the eye can 

 reach. There are alternate beds of sandstone and a steel-brown indu- 

 rated clay. In the second bed of sandstone from the bottom are great 

 quantities of a species of Ostrea. As we pass up in the series we find 

 irregular concretionary beds of rusty calcareous sandstone, with some 

 fossils, especially Ostrea, and a few other marine or brackish-water spe- 

 cies. Everywhere in the West the oldest or lowest coal beds contain 

 more or less marine fossils, most of which belong to the genus Ostrea. 

 They are found, in about the same position, from latitude 49° to New 

 Mexico. Where the beds are studied with some care, in a favorable 

 locality, we soon find that the marine evidences disappear, and the 

 organic remains are purely fresh- water or terrestrial. 



On the morning of October 23 we left our camp on the North Platte 

 and wound across the plains a little north of east to Pass Creek. To 

 the west and southwest, as far as the eye can reach, there is a rolling 

 or partial plain country, occupied by cretaceous beds. Prom the high 

 ridges on the east side of Platte we can cast a glance back along the 

 route we have traveled, over oneof the most comprehensive views I have 

 observed in the West. On the north side is the continuous wall of sand- 

 stone, from Bridgets Pass to Medicine Bow, extending up the Platte 

 River, and retreating with a gentle dip northeast, like descending steps, 

 or rather like chopped waves. The clays underneath the sandstone 

 ridges are undoubtedly of cretaceous age ; but I have been inclined 

 to regard the group of alternate beds of sandstones and clays, which are 

 so conspicuous from Bridger's Pass to Medicine Bow Biver, and give 

 the characteristic surface features to a very large area, as transition 

 beds or beds of passage from the true cretaceous era to the tertiary. 

 It is true that we find here and there a specimen of Inoceramus or Baculite 

 and numerous beds of several species of Ostrea ; yet the time which 

 must have been required to bring about the changes in the sediments 

 and animal life, from a purely marine condition to that of purely fresh- 

 water, must have been immense. That a few of the more hardy marine 

 forms of mollusca should have lingered on up into the period of the 

 coal, would not and need not surprise geologists. So consecutive do the 

 different beds appear to be, that I am of the opinion that, however 

 minutely they may be studied hereafter, the line of separation between 



