HATDEx.] GEOLOGY AGE OF THE LIGNITIC GROUP. 21 



purely fresli-water must reach an aggregate tbickiiesa of 3,000 to 5,000 

 feet. " During the years 1854 and 1855, 1 studie^ this group on the Mis- 

 souri to Fort Benton, and on the Yellowstone, where it is most exten- 

 sively developed, totheiuouth of the Big Horn River and collected great 

 quantities of animal and vegetable remains from the base to the summit. 

 Every season, up to the autumn of 1860, I made collections from this 

 group in all parts of the Northwest. 



The vertebrate remains were studied by Dr. Leidy ; the vegetable 

 fossils, by Dr. Kewberry and Mr. Lesquereux 5 and the in vertebrate fossils, 

 by Mr. Meek and the writer. Kone of us even doubted their Tertiary 

 age. ^Numerous papers were published by Mr. Meek and the writer 

 on the geology and invertebrate paleontology of this region in various 

 journals, but rhostly in the Proceedings of the Academy of Natural Sci- 

 ences at Philadelphia: and inasmuch as these articlesare not easily 

 accessible to the general public, I shall be excused from quoting par- 

 agraphs from them quite freely in an ofiicial report. 



In an article published in the Proceedings of the Academy of Natural 

 Sciences, May, 1857, we state that of the 150 species of Mollusca already 

 described, 54 species are ot Tertiary age, 50 are strictly fresh-water, and 

 only four belong to genera supposed to inhabit salt or brackish waters. 

 This group was even regarded as of Miocene age. The first conclusion, 

 at the close of this paper, reads as follows : — " We have no evidence that 

 any of the Tertiary deposits now known in Nebraska are older than 

 Miocene." 



The above paragraph shows that Mr. Meek and the writer attempted 

 to correllate the various Tertiary groups in the Northwest in the light of 

 the knowledge they possessed at that time. 



But it was from the very abundant fossil flora of this group that the 

 most positive proof of its age was derived. It is hardly ])ossible to 

 estimate with accuracy the thickness of this great group m the North- 

 west, but I should regard it from 3,000 to 5,000 feet in the aggregate, 

 with from twenty to thirty beds or seams of lignite, not including the 

 thin seams of an inch or two, which are very numerous. These vary 

 from six inches to ten feet in thickness. All through this great thick- 

 ness of strata, the leaves are found in most instances in a remarkably 

 perfect state of preservation. Sometimes they are so abundant and 

 so perfectly preserved th^'t they would appear to have fallen from 

 the trees on the spot and in the greatest profusion. It is nut uncommon 

 for a stratum of two feet or more to be composed almost entirely ot 

 these leaves, lying parallel with the layers, as if they had not been dis- 

 turbed after dropping from the trees. Along the immediate vicinity of 

 the main rivers (Missouri and Yellowstone), these plants are the most 

 abundant, far more so than in the more important coal-regions of 

 Wyoming or Colorado. 



Although my own collections, made from 1850 to the autumn of 1860, 

 doubtless comprise the greater part of the species that will hereatter 

 be found, and therefore form a permanent basis for determination and 

 comparison, yet the force of their teachings is somewhat weakened from 

 the fact that the si)ecies from different horizons were not kept suffi- 

 ciently separate. We know, however, that some of the species have a 

 very great vertical as well as horizontal range, and that, so tar as can 

 be detected, there is no break in the sequence of the beds from the 

 Saskatchewan to Santa Fe. 



The following extract is taken from a paper prepared by Mr. Meek 

 and the writer, and published in the proceedings of the Academy of 

 Natural Sciences, Philadelphia, December, 1861. This extract will not 



