106 REPORT UNITED STATES GEOLOGICAL SURVEY. 



Colorado. In both instances the beds belonged to the Potsdam Group, 

 consisting of Potsdam sandstone and the calciferous series, or portion 

 thereof. Dr. Hayden and Dr. Peale have observed the same formations* 

 in the regions examined by them. They, too, refer the beds to the same 

 groups. Dr. Pealet suggests the existence of the Trenton or perhaps 

 even Niagara Group. He places the total thickness of Silurian strata, 

 as observed by himself, at about 820 feet. 



At best, the Silurian of Colorado is of but subordinate importance, 

 as such, in the scale of sedimentary formations. So far as I could de- 

 termine, it has furnished, in a number of instances, the material for 

 metamorphic rocks. Plutonic activity has so thoroughly chauged the 

 original character of the beds, that little more than a guess dare be 

 ventured regarding the former condition of what now we find to be 

 granites, schists, and other kindred rocks. The groups above men- 

 tioned have been determined by fossil remains, and their paralleliza- 

 tion with beds occurring in the eastern portion of the North Ameri- 

 can continent is, therefore, well established. Their occurrence farther 

 north of Colorado and their identification within Dakota, Idaho, Wyom- 

 ing, and Montana, point to a considerable invasion of the western con- 

 tinent during the earliest sedimentary times. It is the Silurian forma- 

 tion, mainly, that can furnish indications as to the distribution of land 

 and water during the period immediately preceding the first well-au- 

 thenticated appearance of life. 



DEVONIAN. 



At a number of localities this formation occurs. It becomes a diflScult 

 matter to draw closely the lines of separation between both the older 

 Silurian and the younger Carboniferous. Essentially we have in South- 

 ern Colorado a very extensive series of limestones, representing the De- 

 vonian formation. Interstrata of shales and sandstones occur near the 

 base and near the top. On the Animas the most complete development 

 was observed. At that locality a considerable portion of the Devonian 

 beds had been subjected to metamorphosing agents, together with the 

 Silurian. I am inclined to the opinion that this has taken place in a 

 large number of instances, comparatively speaking, but at present the 

 original beds have so completely lost their identity that they can no 

 longer be recognized. From the imperfect data that could only be ob- 

 tained with reference tb this formation, it is almost impossible to make 

 any comparison of our Western (Colorado) Devonian with Eastern groups. 

 Should any one be suggested, we may find that the large portion will be 

 parallel to thf* Lower Devonian groups of the East. A prominent feature 

 in the Western Devonian is the intermingling of Devonian with Lower 

 Carboniferous types. This, too, can be observed in those beds usually 

 assigned to the Carboniferous proper. 



CARBONrFEKOUS. 



Into three main groups has this formation been divided. The lowest 

 or oldest one consists of massive beds of limestones. It is followed by 

 a series of sandstones and shales, containing heavy strata of limestone 

 occasionally. A very characteristic red sandstone of great thickness 

 concludes the formation. Within this latter group interstrata of lime- 

 stones occur. They are fossiliferous and have determined the age of the 



" Rep. U. S. Geol. Surv., 1874, pp. 41, 111, and Rep. 1873, p. 202. 

 t Rep. U. S. Geol. Surv., 1871, p. 113. 



