410 



Throughout the Ligniti3 group proper, that is, the portion occurring 

 above the Fox Hills group, I have never found any true nonconformity. 

 That there may be in some places an interrupted sequence in the beds 

 is quite possible, and I am inclined to think the top of the thick bed of 

 sandstone near Point of Rocks is an example. That there have been 

 many oscillations of the surface, that it has been alternately above and 

 below water many times, may be inferred from the numerous coal-beds. 

 In the aggregate, there could hardly have been any marked interrup- 

 tion in the sequence of the deposition, even up to the summits of the high- 

 est Tertiary, though between the Lignitic group and the more modern 

 Tertiaries, as the Washakie, Green Eiver, and other fresh-water groups, 

 there is at this time a true nonconformity. We may conclude that dur- 

 ing the marine Lignitic, the mountain-ranges were slowly rising above the 

 sea-level, yet there was an almost universal sea. During the middle, or 

 brackish-water jperiod, there were huge inland lakes, to which the salt 

 seas had access, thus rendering them brackish, and still the general ele- 

 vation of the crust went on. Still later, these inland lakes were cut off 

 from marine waters, and the deposition of the sediments went on through 

 countless ages, and the mountains continued rising, and these sediments 

 were deposited against their sides as a shore-line. If we could look be- 

 neath the horizontal strata of the Washakie grouj) between Separation 

 and Bitter Creek, or under the Bridger and Green River groups along 

 the immediate line of the railroad, we might find localities where the 

 sequence of the beds is not iuferrupted, and yet, in the immediate vicin- 

 ity of the mountain-ranges, as the Uintah, for example, the modern 

 fresh-water Tertiaries rest uuconformably on the older rocks. The 

 Bridger and Green River groups jut up against the sides of the Uintah 

 range with a very slight inclination. The White River group, on the 

 east side of the Rocky Mountains, shows the same style of unconforma- 

 bility, never inclining more than 5° to 10°, even where the underlying 

 Lignitic beds dip at a much greater angle. 



So far as the geology of the Rocky Mountain region is concerned, the 

 doctrine of the uninterrupted sequence of events is a most important 

 one. If we were perfectly familiar with the structure of every portion 

 of our earth's crust, it is probable that somewhei'e we should find an 

 uninterrupted sequence in the deposition of sediments from the oldest 

 known sedimentary rocks up through all epochs to the present time. 

 We should then have such a unity in the fabric that we would be unable 

 to decide where one age began and another ended; and the terms Silu- 

 rian, Devonian, &c., would only appear as relics of the imperfect knowl- 

 edge of the past. It is the highest aim of the geologist to obliterate, as 

 far as possible, these lines of demarkation ; and if, in the far western 

 regions, we can remove all trace of any chasms between the Cretaceous 

 and the Tertiary epochs, we have made an immense advance toward the 

 completion of the geological fabric. Again, the evidence in regard to 

 the age of the group, derived from the numerous vegetable remains, has 

 been almost entirely ignored by some geologists as worthless, and great 

 stress laid on that obtained from the vertebrates. With all due respect 

 for their opinions, I am confident the reverse is the true state of the 

 case, and that the vertebrates are of little value as tests of the age of 

 these formations. We have shown how readily the change from purely 

 marine waters to brackish and fresh waters would necessarily and did 

 destroy all traces of marine. invertebrate life; yet, according to Professor 

 Cope, Cretaceous vertebrates continued far upward into the fresh-water 

 portions of the true Lignitic group of the Northwest. Not a single species, 

 however, seems to be identical with any found in other portions of thje 



