12 SUEVEY OF COLORADO AND NEW MEXICO. 



along tlie margins of the mountains, but all rocks of more recent date 

 are concealed by tbe tertiary deposits. In order that I may make my 

 description of tlie different formations in their southern extension more 

 clearly understood, I will describe them in as brief a manner as pos- 

 sible, as they have been studied in the regions to the northward. 



The granites and metamorphic rocks do not differ in many respects from 

 those which form the nucleus of the mountain ranges generally. Eeddish 

 and gray granites form the central portions, and on the sides is a series of 

 stratified metamorphic rocks of a great variety of structure and com- 

 position. At the north the igneous rocks do not seem to predominate 

 in the eastern ranges, but as we proceed southward toward New Mexico 

 they increase in extent and force. 



The Potsdam sandstone is the only member of the silurian that I have 

 ever observed along the margins of the mountains. It was first dis- 

 covered west of the Missouri Eiver in the summer of 1857, during the . 

 exploration of the Black Hills of Dakota, by a United States expedi- 

 tion under the command of General G. K. "Warren, United States Army, 

 and it has been observed in several other localities since that time. 



The following section of the Potsdam sandstone in its relation to the 

 carboniferous beds, as observed by me around the margins of the Black 

 Hills, shows the typical characters of each, where they are weU exposed 

 and have been clearly identified by organic remains : 



1. Hard, compact, fine-grained, yellowish limestone of an excellent 

 quality; passing down into a yellow calcareous sandstone, quite friable. 

 Fossils : Bhynconella rocJcy-montana, Athyris suhtilita, Cyrtoceras, &c. — 

 50 feet. 



2. Loose layers of very hard yellow arenaceous limestone with a red- 

 dish tinge, underlaid by a bed, six or eight feet in thickness, of a very hard 

 blue limestone. The whole contains great quantities of broken crinoi- 

 dal remains with cyathopylloid corals and several species of brachio- 

 poda — 40 feet. 



3. Variegated sandstone of a gray and ferruginous reddish color, com- 

 posed chiefly of grains of quartz and particles of mica, cemented with 

 calcareous matter. Some portions of the bed are very hard, compact, 

 siliceous; others a coarse friable grit; others conglomerate. Fossils: 

 Lingula ])r%ma^ L. antiqna, Oholella nana, and Ariofiellus oweni — 50 feet. 



4. Stratified metamorphic rocks in a vertical position for the most part. 

 Eocks about the same as those above described, sometimes very much 



thicker and sometimes thinner, have been seen, more or less, all along the 

 margin of the Eocky Mountains, on both sides the main axis from the 

 north line to Cheyenne. 



About the sources of the Missouri Eiver, along the flanks of the Big 

 Horn and Wind Eiver Mountains, these rocks are particularly devel- 

 oped. Now and then they all disappear for a considerable distance, and 

 then, at the first favorable opportunity, reappear from beneath beds of 

 more recent date. A series of arenaceous beds, which we have called 

 the " red arenaceous deposits, or triassic," form one of the most con- 

 spicuous features of the geology along the flanks of both sides of the 

 principle ranges of mountains and are almost always present. They 

 were first observed by me, forming a narrow belt or girdle around the 

 granite nucleus of the Black Hills of Dakota, in the summer of 1857. 

 These rocks are sometimes called saliferous or gypsum-bearing beds, from 

 the fact that they contain both salt and gypsum, the latter mineral 

 oftentimes in great quantities. There are also mingled with these beds 

 several layers of bluish siliceous limestone, which at the far north at- 



