HATDEK.] GEOLOGY OF COLOEADO SPEINGS AND PLEASANT PARK. 41 



extent on both sides of the eastern ranges that front the plains, from 

 the northern to the southern boundary of the United States, and how 

 much farther I do not know. It is probable, however, that they extend 

 far north into the British Provinces and far south into Mexico. This 

 belt is very varied in its character from point to point, sometimes ex- 

 pands to a width of several miles, and again contracts to a fourth or 

 half a mile in width. Sometimes a full series of the formations known 

 in the west, from the older Silurian to the most modern Tertiary, are 

 clearly exposed, inclining at various angles ; and then again only the 

 more modern beds can be seen. It is on this account that the geology, 

 though appearing so very simple in its character, is really quite com- 

 plex when examined in detail. 



Up to this time we have determined the existence, in this belt in 

 Colorado, of the Silurian, Carboniferous, Triassic,(?) Jurassic, Cretaceous, 

 and Tertiary groups; yet, while the more modern formations are very 

 persistent throughout the entire distance from the north to the south 

 line, some of the older beds are wanting in many places. To the far 

 north, along the margins of the Black HiJls, Big Horn, and Wind Eiver 

 Mountains, the Potsdam sandstones, with perhaps more modern divis- 

 ions of the Silurian, are well exposed and quite continuous, while to the 

 southward these rocks disappear, except at restricted localities. We 

 find near Colorado Springs and Canon City quite large exposures of 

 Silimau beds, with a few fossils that are allied to those of the Calcifer- 

 ous group of the Lower Silurian of New York. In the interval, from 

 Fort Laramie to Colorado Springs, a distance of over 200 miles, no one 

 connected with the survey under my charge has yet detected imy trace 

 of these beds. It is possible that in Pleasant Park, about 50 miles south 

 of Denver, there are traces of this formation in the variegated sand- 

 stones that lie next to the granites, as shown in the section. At Color- 

 ado Spiings, and in the vicinity, there is a considerable thickness of the 

 Silurian beds, which have been frequently described. The reddish- 

 brown, rather coarse sandstones at the base, rest upon stratified grani- 

 toid or gneissic rocks unconformably, as shown in the illustration (PI. VI, 

 Fig. 2). Above the sandstones there are 600 to 800 feet of yellowish 

 limestone, which, in some instances, is a reddish color, in which have 

 been found several species of invertebrate Silurian fossils. South of the 

 valley of Fountain Creek the uplifted belt rapidly closes up to the base 

 of the mountains, and for some distance no beds older than the Creta- 

 ceous are visible. From Fountain Creek to Caiion Cit^' this belt ex- 

 pands and contracts from time to time, so that it is quite possible that 

 small isolated patches of the Silurian group may appear in a few places. 

 At Canon City, and resting for the most part on the mountainsides, in- 

 clining at a high angle, there is an extensive thickness of these older 

 beds again. The lower portion is a variegated micaceous, slightly cal- 

 careous, sandstone, closely resembling, in texture and composition, the 

 Potsdam sandstone as seen in other localities farther to the north. 

 Some tolerably well-defined fossils were discovered in the sandstone 

 which rests directly on the granitic rocks, which Mr. Meek has pro- 

 nounced of undoubted Lower Silurian age, but they have not yet been 

 described. This sandstone passes up into a hard and rather massive 

 limestone, evidently the same as that noted at Colorado Springs. 



We cannot say more at present in regard to rocks of this age, than 

 to state our belief that they underlie the entire country along the east- 

 ern slope of the Eocky Mountains, from our northern boundary to New 

 Mexico, and that, where they are not visible, they may possibly be con- 

 cealed by the overlying and more modern beds. 



