ENDLicH.] GEOLOGY SAN LUIS DISTEICT, -SECTION C. 339 



within the former, keeping approximately parallel with the westerly dip 

 of the euphotide. Descending from the peak, this rock is suddenly cut 

 off in an almost vertical, sharp liue, visible for nearly 2,000 feet along 

 the side of the mountain, and granite sets in, which continues until the 

 yalley is nearly reached. Here we find the Silurian beds, (c,) perhaps 

 even Carboniferous, although that formation was not identified with cer- 

 tainty, dipping off toward the river. Drift-deposit (d) obscures the struc- 

 ture entirely, until the low rolling hills on the north of the Arkansas 

 show granite exposed. Without much variation in altitude, this contin- 

 ues for about ten miles, until a bluft", rising rather abruptly, is crowned 

 by the Silurian strata, (c,) upon which station 55 was located. The very 

 formation of this bluff is probably owing to the hard, protecting cap of 

 quartzites and limestones. It seems reasonable to suppose that the beds 

 we found at the base of station 2 were at one time connected with 

 those twenty miles farther to the northeast, and uuder circumstances 

 whereby the construction of connecting-lines would indicate the forma- 

 tion of an S-shaped fold, which may have served to break up and destroy 

 the connection. There seems to have been an inequality in the rising 

 of the land on the two sides of the Arkansas "at that point, which may 

 account for the fold. This view of a former connection is strengthened 

 by the fact that but fifteen miles below our section the Silurian of both 

 sides approach each other within four miles. In the northwest corner of 

 section c, the Silurian was also found, and identified again by its petro- 

 logical character and its normal position to the Carboniferous. As it 

 conducted itself, so far as could be noticed, throughout conformably 

 with the latter, and no special stratigraphical studies could be made re- 

 garding it, I will refer to the descriiDtion of the Carboniferous of that 

 region. A doubtful point occurs at stations 3G and 37, where the quartz- 

 ites closely resemble those of the Silurian formation ; but the absence of 

 Carboniferous in the line running from that point to the Cretaceous 

 strata, without any inversion having taken place, prevents their exact 

 identification, and therefore that point shall be left doubtful. 



The Carboniferous formation occurs at one point only in the eastern 

 portion of section c, unless it might be found overlying the Silurian, 

 where it was not identified, however, lining the outer edge of the gran- 

 itic mountains bordering upon Homan's Park. 



In speaking of the Sangre de Christo range, reference has already 

 been made to it, and the probability suggested, which is corroborated 

 by this occurrence, that the Arkansas sandstone is persistent across the 

 northern jjortion, and perhaps partially so across some of the middle 

 and southern portions of San Luis Valley. The dip here is to the east, 

 away fr©m the mountains it rests upon. Of more importance are the 

 Carboniferous rocks of the northwest portion of section c. Leaving 

 stations 30 and 37 as undecided, we find farther northward the highlands 

 north and south of the canon of the Gunnison composed of Carbon- 

 iferous limestones of considerable thickness. Ascending the steep 

 granitic hills forming either side of the canon, the stratified sedimentary 

 rocks are seen crowning the top and gently dipping toward the caSon. 

 Station 38 is located on a peak of eruptive granite, and surrounded on 

 all sides by sedimentary beds. The northern portion of section B, Plate 

 XIV, will show more clearly the position. After leaving the coarse- 

 grained red granite, (a,) an immense dike of dioritic rock occurs, strik- 

 ing considerably out of course, almost east and west ; but immediately 

 after passing this dike, a heavy mass of limestones and quartzites (c) is 

 found, upon the edge of which station 38' was located. These strata 

 have a dip to the south of about 20° to 25°, increasing, however, as we 



