ENDUCH.] GEOLOGY SAN LUIS DISTRICT, SECTION A. 321 



east. It is of white color, with sanidite, oligoclase, black mica, and iso- 

 lated crystals of hornblende. 



The trachorheites in section a have a main strike of about 10° north 

 of east. They form numerous little volcanic islands, owing to the 

 weathering' of their own and the under-lying strata. 



Besides the trachorheites, and i)erhaps one or two points of propylite, 

 which are doubtful to me, however, I have not been able to observe any 

 other volcanic rocks iu section a. There remain to be discussed yet 

 the dikes, drift, and ore-deposits. 



A large number of dikes, running from north to south, traverse the 

 granite of the section under consideration, varying in breadth from 2 to 

 100 feet. In most cases they present a dioritic composition, consisting 

 mainly of feldspar and hornblende. The latter occurs either dissemi- 

 nated in fine grains throughout, or radiated or segregated iu long, poorly- 

 developed crystals, or, particularly toward the two outer sides of the 

 dike, the hornblende assumes a schistose character. Mica is rarely want- 

 ing. Quartz occurs quite frequently, although not entering into the 

 mineral composition sufficiently to change its character. Epidote almost 

 invariably occurs in small light-green crystals, lining small fissures and 

 cavities that may have formed. It seems probable, from the compara- 

 tively small extent of these dikes, that they owe their existence to infil- 

 tration and segregation in preference to injection, although in some 

 cases the latter may be true. They will be found more frequently on 

 the crests of mountains and hills than at any other part. Drift we have 

 none in section «, except a few very small patches along the Arkansas 

 Eiver — merely a deposition of pebbles. No deposits of ores were found 

 in this section. 



Be8ume of Section A. — As far as could be determined, little if any dis- 

 turbance took place in section a before the closing of the Carboniferous 

 period. The eruptive activity that upheaved the Sangre de Christo range 

 in section h reached over into sec^tion a, occurring probably at a period 

 shortly after the deposition of the Carboniferous formations. The 

 Arkansas sandstone in the western part of the section, as well as the 

 limestone under- and overlying it, owe their northeasterly dip to 

 the action above mentioned. At the adopted end of the Cretaceous 

 period the entire Front range began to rise very gradually, not rising 

 equally iu the entire mass, but faster aloug a line well marked by the 

 present boundary of the granite. During that rise a sufficient amount 

 of land was exposed to allow the formation of the lignitic group to go 

 on at favorable points. This gradual elevation seems to hav^e been ac- 

 celerated at certain points a little more than at others, although it was 

 comparatively very uniform. By this means the beds reposing on the , 

 gianite were gradually raised ; they may have been partly broken, so as 

 to be carried away more readily b\^ erosive action, but probably in no 

 instance did they cover much more of the granite than at present. Dur- 

 ing the deposition of the older Tertiary beds this gradual rise seems to 

 have been still going on, so that an uuconlbrmability that might be ex- 

 pected is more or less obliterated. The extent of the motion was not 

 very far inland, but, as mentioned above, occurred along a line running 

 parallel with the present boundary of the granite. Although antici- 

 pating, I will state a feature observed in section b that may serve to 

 21 G s I 



