210 REPORT UNITED STATES GEOLOGICAL SURVEY. 



tbe crystals weather out very readily. Some portions of the porphyry 

 contain minute crystals of hornblende. All of these dikes are in con- 

 tact with either granitic or gneissic rock. No marked regularity of 

 course can be observed. 



Taking a general view of all these basic plutonic dikes, they are found 

 to have a number of features in commoa. Usually they occur without 

 any apparent provocation, several of them form a small group, and each 

 member of this group agrees, in its characteristics, with the others. As 

 a general thing their strike may be said to be approximately north and 

 south. Their width is variable, from a few feet to several hundred. 

 Accessory minerals, such as feldspar, garnets, small crystals of mag- 

 netite, pargasite, and others are contained within the dike-rock. With 

 the exception of the one instance, all these dikes are associated with 

 presedimentary rocks. Their age, therefore, may be regarded as an 

 open question. My impression is that the diorites are oldest, the por- 

 phyries youngest. 



ACIDIC PLUTONIC ERUPTIVES. 



Granite. — In the northern part of the Sangre de CristoEange an erupt- 

 ive granite occurs.* It is readily distinguished from the older, meta- 

 mor])hic. Orthoclase is light gray, white, and yellowish ; small crystals 

 of oligoclase are dispersed throughout; quartz is colorless or milky 

 white ; the mica black, sometimes in small six-sided crystals. The ap- 

 pearance of this granite upon the surface was accompanied and proba- 

 bly produced by the demonstration of powerfully-acting vertical force. 

 Carboniferous Jbeds of great thickness were thrown upward so as to 

 form a steep, anticlinal range. In the centre of this we find the granite. 

 Its relative position to the sedimentary beds is constant throughout. 

 No cases of intrusion were observed, or even the formation of dikes 

 within tbe unchanged sedimentaries. A considerable amount of meta- 

 luorphism has taken place, totally changing some of the oldest strata. 

 North of Hunt's Peak and south of Mosco Pass the eruptive granite 

 again is lost, and only metamorphic granites are found in other |3ortions 

 of the range. From the entire character of the range it would appear 

 that the disturbances there were not owing, directly, to the eruption of 

 the granite, but that the latter thereby found room to escape upward. 



The most important and typical occurrence of eruptive granite in 

 Colorado was observed in the Elk Mountains. Along the line of a defi- 

 nite axis this range has been elevated, appearing " to be an example of 

 a sudden, violent, or catastrophic action."t Granite forms the central 

 mass, the underlying bulk of the range, while the sedimentary beds, 

 many thousand feet in thickness, have been contorted, overturned, and 

 broken. Mr. Holmes has published | a very excellent diagram, repre- 

 senting the exposure of eruptive granite along the main axis of the 

 range. From this it appears that the main exposures of eruptive granite 

 occur in the Snow Mass and in the White-Eock groups. Between these 

 two the granites are hidden from sight by the contorted sedimentary 

 beds. These form an S shaped fold here, overlapping from the east- 

 ward. Probably the large masses of sedimentary strata were thoroughly 

 broken, and finally removed from the localities where now the granite 

 is exposed. Dr. Hajden says,§ " This is a grand illustration of an erup- 

 tive range." In spite of the violent disturbances to which the sedi- 

 mentary strata have here been subjected, the range presents essentially 

 the type of an anticlinal. Acting along a definite axis, the trend of 



* Rep. U. S. Geol. Surv., 1H7:5, p. 324. t Ibid., p. 70. 



tRep. U. S. Geol. Surv., lb? 4, p. 55. § Rep. U. S. Geol.Surv., 1874, p. 5.5. 



