KNDLi.H.] GEOLOGY SAN LUIS DISTRICT, SECTION C. 335 



CHAPTER IV. 



GEOLOGY OF SECTION C, SAN LUIS DIVISION. 



Section c is bordered on the east by the Arkansas Eiver, Poncho Pass, 

 and San Luis Valley, while the southern, western, and northern limits 

 are the same lines that inclose the entire district. Altogether the coun- 

 try of this section presents a mountainous aspect, containing, with a 

 high average elevation, a number of our highest peaks, and those por- 

 tions of the main continental divide that fall within the boundaries of 

 our district. The southwestern portion, to an extent of more than 1,400 

 square miles, is one continuous mass of trachorheites, with all their 

 characteristic orographic features. Toward the foot-hills, and near tne 

 few wider valleys that occur therein, frequent formations of table- 

 mountains are found, sometimes only small, sometimes of considerable 

 size; and the ditferent flows of trachoid rocks have i^roduced mountains 

 with well-marked steep terraces. To the northeast of this immense 

 volcanic area we get into granite and hornblende rocks again, forming 

 massive, heavy mountains, skirted by lower foot-hills composed of sedi- 

 mentary rocks, which lead oft' into the valley of the Arkansas. West 

 of this valley steep masses of granitic mountains rise to an altitude of 

 over 6,000 feet above valley-level, almost ideal in shape, certainly pre- 

 senting an imposing sight. These sharply-cut mountains vary some- 

 what in the character of the material of which they are composed, much 

 less in their general outlines, however. Northwest of them is the 

 deep caSon of the Gunnison, bordered on either side by steep, high 

 walls, with a usually very narrow surface in the valley itself. Leav- 

 ing the caQon at the western terminus of our district, we find the 

 aspect of the country has suddenly grown much more gentle ; the influ- 

 ence of sedimentary beds is again noticeable. A long valley, about five 

 to seven miles in width, extends over about eighteen miles to the south- 

 east, fairly studded in its upper half with small box-shaped bluffs, com- 

 posed of Cretaceous strata. 



Granite covers a large portion of the area in section c, but is sur- 

 passed in quantity by the trachorheites. In the northeastern part it runs 

 parallel to the Arkansas, keeping about five miles distant, borders Pou- 

 cbo Pass, and extends southward for eight to ten miles, but is soon 

 covered by volcanic rocks, and appears in the southwestern corner only 

 in small isolated patches exposed by erosion. In the northwestern ex- 

 tension from the Arkansas it is the main rock, but is covered partly by 

 sedimentary strata, partly obliterated by volcanic eruptive rocks ; prob- 

 ably it underlies quite a considerable portion of the trachorheites of the 

 southwest, and, as far as could be judged from a very distant view, 

 comes in again farther westward. In this section, as in the preceding, 

 there are also two distinctly detined varieties of granite, one evidently 

 younger than the other. Beginning with the granite immediately west 

 of Poncho Pass, we find it to be very much of the same character as 

 given in the x)receding section; it is subject to a number of variations 

 and modifications, caused by local accumulations of one or the other 

 constituent mineral. If the mica predominates, in proportion it will 

 change the structure of the granite, and all varieties from typical gran- 

 ite to equally typical gneiss can be observed. In texture it varies from 

 a very fine-grained granite loosely put together to one yielding tablets 



