W. B. Emery— Igneous Geology oj Carrizo Mountain. 361 



Name. — The mineral character of this rock is so clearly that 

 of a clacite, that, even without chemical analysis, it has been 

 classed as such. 



Manner of Intrusion. 



Daly has recently classified all intrusive igneous masses 

 under two main heads, (1) injected and (2) subjacent bodies, 

 according as the} 7 come into place either by the injection and 

 consequent uplift of the enclosing strata or by the replace- 

 ment of these beds. Under the first division are placed sills 

 and laccoliths ; under the second, stocks and batholiths. 



The uniform character of the diorite porphyry of Carrizo 

 Mountain demonstrates that if, as is possible, the various out- 

 crops of that rock are not portions of one large intrusion, they 

 at least represent upwellings of magma from a single common 

 reservoir. Nowhere were there observed stoped blocks nor 

 was any evidence of assimilation noted, facts which, taken 

 together with the upturning of the beds at the base of the 

 mountain, suggest that intrusion was by injection rather than 

 by a replacement of surrounding beds, — that the intrusion is of 

 the injected rather than of the subjacent type. 



It is evident from a study of the writings of Holmes and 

 Peale that they both considered the Carrizo intrusion as belong- 

 ing to the class of intrusion to which the name laccolith was 

 later applied by Gilbert. Indeed on the geologic sections 

 accompanying the maps of the Hayden Survey, Carrizo Moun- 

 tain is portrayed in laccolithic form.* !No sedimentary floor 

 is as yet exposed by erosion and that this intrusion is sym- 

 metrical and of typically laccolithic shape cannot be assumed. 

 Aside from the irregularity produced by Tisnasbas sill and the 

 sheets capping North and Chezhindeza mesas, it seems clear 

 that the intrusion is asymmetrical and has in places broken up 

 through the overlying beds. The porphyry may be seen cut- 

 ting across the Triassic rocks and the Wingate sandstone 

 (Jurassic) in Tisnasbas canyon to supply the material for the 

 Tisnasbas sill. It also appears that the igneous mass east of 

 Biltabito Canyon has cut across the Triassic rocks and the 

 Wingate sandstone, and there are probably other transgressions 

 not yet exposed by erosion. Indeed it seems possible that the 

 intrusion consists of several sills with their connecting pipes 

 which have united in producing the effects of laccolithic uplift. 

 Until such facts can be definitely proven, however, it is at least 

 convenient to speak of the intrusion as a laccolith, which 

 form of intrusion to all outward appearances it most closely 

 resembles. (See fig. 5.) 



* Hayden, F. V., U. S. Geol. and Geog. Survey Terr., Atlas of Colorado 

 and portions of adjacent territory, 1877, Sheet XVII, Section 11. 



