EXDL1CH.J GEOLOGY SAN LUIS DISTEICT, SECTION C. S4S 



in the form of a bay, and was probably driven back by the volcanic ac- 

 tivity that was developed soon after south of it. About ninety square 

 miles are covered by this formation here ; and if it extends under the 

 trachorheites as far as I think it probable, it will amount to about one 

 hundred and fifty square miles more. Between stations 26 and 27 a small 

 fresh-water deposit was found, undoubtedly belonging to the Tertiary 

 period, and probably the Miocene, A section on page , illustrating 

 the position of the eruptive volcanic rocks, will show its position. Eest- 

 iug apparently upon granite, it dips off gently to the westward, contain- 

 ing, in its strata of white argillaceous limestones and shales, numerous 

 remains of plants and smal shells of Limncea. The volcanic material 

 coming from station 27 eastward covered the greater portion of theiittle 

 bay, so that not much of it is exposed at present. This occurrence may 

 be some small, merely local, fresh-water deposit, or it may be the continua- 

 tion of a more extensive series of strata, which cannot be determined un- 

 less the country is either very well known, so that additional exposures 

 might afford some reliable data, or otherwise artificial means of testing 

 wouldbeemployed. Of thetwo, theformer seems more probable, as some 

 of the neighboring streams cut in quite deeply, down to the granite, so 

 that any sedimentary rocks occurring there would have been exposed. 

 This little outcrop is the only one of Tertiary beds that was found through- 

 out the entire section, but it seems probable, judging from the orographic 

 features of the country, that west of our district we may find more exten- 

 sive formations belonging to that age. It may seem strange that along the 

 center and consequently western terminus of San Luis Valley there is 

 neither Cretaceous nor Tertiary, as the waters of tbat period could not 

 have had any very great obstacles to overcome in reaching those points. 

 The larger portion of section c is covered b^^ volcanic rocks belonging 

 to the trachorheite group, which form a heavy mass of mountains, 

 sometimes rising to a very considerable elevation, covering at the same 

 time over fourteen hundred square miles. 



y In this main mass of trachorheites we find every variety belonging to 

 the group represented. As a rule, it may be said that the highest and 

 most prominent points are comi)osed of andesites and trachytes, the 

 latter rarely, however j that the plateaus and terraces are andesitic, 

 sometimes trachytic tuffs ', the low, bluff'-like hills along the base of the 

 higher mountains are of a rhyolitic character ; and a number of little 

 hillocks are formed by an ashy material, probably not much older, if not 

 as young, as the rhyolites. Discrimination from the lithological char- 

 acter alone becomes very difficult wherever such an immense number of 

 varieties occur. On every hill, on every station, several varieties of the 

 predominating rock can be found, varying within twenty yards of each 

 other to such an extent that it is by no means an easy task to draw the 

 lines of distinction correctly. A mass of high andesitic mountains occurs 

 about five to six miles south of Mount Ouray, upon one of which station 

 24 is located at an elevation of 13,400 feet, and throughout that portion 

 of the volcanic country a number of peaks nearly as high occur, forming 

 an almost regular horseshoe, studded with numerous smaller hills inside. 

 When seen in the field, the impression produced by it was that of one 

 huge crater-edge, containing within its limits a number of smaller eruptive 

 cones. Along the crest of that crater, the andesites are very compact, 

 hard, almost jaspery in appearance, changing somewhat as they descend 

 into the lower jjortions ; and on the eastern edge those mountains are 

 bordered by compact, red rhyolites that have flowed down into the 

 valley ^o an elevation not much over 7,500 feet. Andesitic tuff's make 

 up a very considerable portion of the country there, but, contrary to 



