354 W. />'. Emery — Igneous Geology of Car ri::o Mountain. 



group of radiating dikes. In these dikes the rock is soft yet 

 more resistant to erosion than the enclosing sedimentary beds, 

 so that in every ease the dike stands up as a wall of greater or 

 less height. 



Volcanic plugs. — A series of six volcanic pings is exposed 

 southeast of Carrizo Mountain. When these are joined 

 together they are seen to be arranged on a very flat reversed 

 curve about 20 miles long. Such linear arrangement suggests 

 the presence of a fault as a line of weakness favorable for 

 intrusion, but no movement of that character was recognized. 

 These pings are rather prominent features of the landscape, 

 standing out dark against the red or light-colored sandstones at 

 their base. They are all of the same character, Walker Peak 

 (fig. 4), the southernmost one, being typical, rising conical at 

 the base but terminated above by cliffs 100 or more feet high. 



The Petrography of the Intrusions. 

 Diorite Porphyry. 



Occurrence. — The central mass of Carrizo Mountain is of 

 diorite porphyry, as are also the sheet-like intrusions associated 

 with it. The porphyry, which is exposed in all the canyons 

 and in many other places on the mountain summits, covers an 

 area of over 100 square miles. Holmes, in speaking of the 

 intrusions and the mountain resulting from them, says 



"It is a typical example of the eruptive groups of this part of 



the Colorado plateau It has a nucleus of its own, and so 



far as the surface is concerned is independent of all other erup- 

 tive masses The trachytes [diorite porphyry of this 



report] are now found chiefly in contact with the Lower Cre- 

 taceous [now referred to the Jurassic] and Jura-Trias rocks, for 

 the reason that the Middle Cretaceous shales, in which a large 

 part of the trachyte was originally deposited, have been com- 

 pletely carried away, leaving only small fragments imbedded in 

 the faces and upper surfaces of the trachyte."* 



Ifacroscopic description. — In the hand specimen the dio- 

 rite porphyry shows an abundance of plagioclase phenocrysts 

 and less numerous prisms of hornblende set in a white to 

 grayish, aphanitic gronndmass. Quartz phenocrysts are not 

 uncommon. With regard to the megascopic character of the 

 rock Holmes says : 



"A specimen of trachyte from West Mesa is found to resemble 

 closely in appearance and composition the trachyte of the other 

 groups of the southwest. It has a bluish white paste, which con- 



* Holmes, W. H., U. S. Geol. and Geog. Survey Terr., embracing parts of 

 Colorado and adjacent territory, 1875, p. 274. 



