sr.romr.J MT. LEIDY HIGHLANDS. 457 



the name of Mount Leidy, in honor of the distinguished comparative 

 anatomist, and which may be used to designate the well-defined high- 

 land region of which it forms so prominent a topographic feature. For 

 so moderately elevated a region its surface features are exceedingly 

 broken and diversified. The plateau-like uplands are cut by narrow 

 drainage channels, and the higher ridges are carved into the most intri- 

 cate forms by the erosive action of atmospheric agents. These features 

 offer marked contrast to the massive mountain topography of the region 

 we have just left, and this finds ready explanation in the equally pro- 

 nounced contrast that subsists in the geological features peculiar to each. 

 The whole region is pretty well covered with forests of pine and fir j 

 indeed, extensive tracts of dense timber are found on the gentler slopes, 

 and equally large areas of burnt forests are grown up with an almost 

 impenetrable growth of young pines. The upland levels are also thickly 

 covered with groves of aspen, interspersed with open grazing-lands. 

 The steeper mountain summits are generally perfectly nude, while the 

 lower ridges show frequent hues of bluffs in which the strata are revealed 

 over almost the entire field. Fortunately this circumstance was greatly 

 in our favor, but for which, indeed, we should have brought away even 

 less satisfactory data than what rewarded the very hasty examinations 

 which we were compelled to bestow upon the region. 



We have already seen that the mountain range to the south of this 

 area presents a barrier of uplifted sedimentary formations composed of 

 Palaeozoic and in part of Mesozoic strata, which dip northward, the 

 older deposits indeed showing only a limited area of outcrop north of the 

 Gros Ventre Elver in the vicinity of its debouchure on the border of 

 Jackson's Basin. The Mesozoic formations, however, in the lower course 

 of the Gros Ventre Valley, at least, show their full development in the 

 immediate vicinity of the stream, extending back, northward, where 

 they occupy a narrow belt in the southern flank of the Mount Leidy 

 highlands ; but to the eastward they, too, reach southward, and at least 

 the " jed beds" are carried high up on the northern flank of the Gros 

 Ventre Eange, as has been already observed. 



With regard to the nature of the geological formations that occur 

 toward the sources of the Gros Ventre, the observations made by Dr. 

 Hay den, in i860, have so pertinent a bearing on the subject under 

 present consideration, that the liberty is taken to transcribe in full what 

 he has recorded on the geology of that section of the country. Under 

 date of June 5, he writes:* "We ascended a high ridge, from which 

 we could see to a great distance. Looking to the dividing crest of the 

 Wind Eiver Mountains, we find the exposed belt of granite to be not 

 more than four or five miles in width, and gradually lost in the basaltic 

 or eruptive range, which also renders itself conspicuous. The Tertiary 

 beds seem to reach fully up to the crest on the west side [in the vicinity 

 of Union Pass], and often passing * * * even to the entire divide 

 of the mountains. We also see, high up on the flanks of the mountains, 

 a full series of the more recent Tertiary beds, with pinkish bands, pre- 

 cisely similar to those in the Wind Eiver Valley. These pass up into 

 yellow sandy marls and sandstone. I have estimated the entire thick- 

 ness of the Tertiary beds on the west side of the mountains at 1,200 to 

 1,500 feet. In the lignite beds and vicinity are great quantities of 

 selenite and silicified wood. All over the highest hills near the crest of 

 the mountains, 10,000 feet above the sea, are the recent Tertiary beds. 



* Exploration of the sources of the Missouri and Yellowstone Eivers, under the 

 direction of Capt. W. F. Eeynolds, Topographical Engineers; Geological Report by F. 

 V. Hayden, 1869. Eeprint U. S. Geological Survey, 1872, chap. I. 



