W. B. Emery— Igneous Geology of Carrizo Mountain. 357 



One inclusion, however, in the hand specimen, was seen to 

 fade gradually into the enclosing porphyry. Study of a thin 

 section, containing a portion of both the including and the 

 included rock, revealed the fact that in each there were phen- 

 ocrysts of similar common green hornblende set in a similar 

 groundmass. The inclusion possessed the characters of the 

 main mass, developed on a smaller scale. This has led the 

 writer to conclude that the inclusions represent portions of the 

 magma, previously solidified, which by later movement came 

 into place in Carrizo Mountain. An occurrence, very similar 

 to this, of hornblende inclusions in diorite porphyry, has been 

 described by Iddings from Electric Peak in Yellowstone 

 National Park.* 



Classification. — Holmes was the first to study the rock of 

 the intrusions of Carrizo Mountain. Like his coworkers in the 

 Southwest at that time, he speaks of this rock in the text of 

 his report as a " trachyte."f However in the legend of the 

 geologic map accompanying the reports of the work done by 

 the Hayden Survey in Colorado this same rock is listed as 

 " porphyritic trachyte (hornblendic)."^: Cross has later had 

 occasion to study the very specimens collected by Holmes from 

 Carrizo Mountain, together with the rocks of other laecolithic 

 intrusions of the Southwest, brought in by the Hayden geol- 

 ogists. He has found that new names must be applied to 

 these rocks, and in speaking of this he says 



" In describing these rocks, ... it will be necessary to use a 

 nomenclature almost entirely different from that adopted by 

 Gilbert, Dutton, Holmes and Peale. That this is true is not a 

 reflection upon these able geologists, for the modern science of 

 petrography was unknown in this country at the time their work 

 in these regions was done. Few of the specimens collected by 

 them had been examined microscopically or chemically when the 

 published reports were written and it is usually stated in those 

 reports that the names used are adopted provisionally."! 



Accordingly, in place of Holmes' name, "trachyte," Cross 

 applied the term hornblende porphyrite to the rock of Carrizo 

 Mountain. Since his writing, however, the name porphyrite 

 has itself been abandoned by American petrographers, and this 

 type of rock is now known as diorite porphyry. It should be 

 noted, however, that while the rock is classed as a diorite por- 

 phyry, there is in places considerable quartz present. 



Place in the Quantitative Classification. — A chemical anal- 

 ysis of a rock which, from his description, is thought to be the 



* Iddings, J. P., The Eruptive Rocks of Electric Peak, 12th Ann. Kept., 

 U. S. Geol. Survey, Part I, 1890-91, p. 597. 



t Holmes, W. H., op. cit. 



JGeol. and Geog. Atlas of Colorado and portions of adjacent territory, 

 Hayden, 1877, Sheet XV. 



§ Cross, Whitman, The Laecolithic Mountain Groups of Colorado, Utah, 

 and Arizona. 14th Ann. Rept., U. S. Geol. Survey, Part II, 1895, p. 175. 



