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



Depth of Cover. 



The lowest formation observed in contact with the porphyry 

 of Carrizo Mountain is of Triassic age ; the highest is the 

 McElmo formation (Jurassic?) within which the Tisnasbas sill 

 is intruded. It is evident from a study of the surrounding 

 region that the Cretaceous sediments were once continuous 

 over the area, but that they were present at the time of intru- 

 sion remains to be demonstrated. Indeed the determination 

 of the depth of cover depends upon the age of the sandstone 

 and conglomerate which cap the mountain summit and outcrop 

 for a short distance on the mountain flank south of Chezhin- 

 deza Canyon. This sandstone, which is similar both to the 

 Dakota and to certain Tertiary sandstones and whose age cannot 

 be determined because of absence of fossil evidence, rests in 

 angular conformity and apparently without erosional uncon- 

 formity upon the Triassic rocks. Whether Cretaceous or Ter- 

 tiary, an erosion cycle is necessary to bring the Triassic into 

 juxtaposition with formations so much higher than it. It is 

 known that in northern Arizona there were such erosion cycles 

 in both pre-Dakota and pre-Tertiary time, so that erosion is of 

 no avail in the age determination of this intrusion. At present 

 it can only be said that if the beds are Tertiary, intrusion took 

 place below a cover of 2000 feet ; if the beds are Dakota, 

 beneath a cover of about 5000 feet. 



Age of Intrusion. 



The age of the various laccolithic intrusions of the Southwest 

 has been considered by all students of that region as Tertiary, 

 but so far as the writer has been able to ascertain, little definite 

 evidence of this has been adduced. Were it possible to deter- 

 mine the age of the sandstone capping Carrizo Mountain, the 

 date of intrusion there might be very definitely ascertained. 

 Even in the absence of such proof it is still possible to place 

 the age of the intrusion with much assurance as Tertiary, 

 though not more definitely than that. It is known that there 

 are certain structures of pre-Tertiary age in the area, and that 

 these structures have been magnified by intrusion. Since, then, 

 intrusion is younger than these features, which were probably 

 developed at the end of the Cretaceous, it must have occurred 

 in Tertiary time. 



The relative ages of the different intrusions, the main mass, 

 the sills, sheets, volcanic plugs, and dikes cannot be determined 

 as they were not observed cutting each other. 



Contact Metamorphisrn. 



Tisnasbas sill. — There are only a few contacts of the diorite 

 porphyry and the enclosing sedimentary rocks visible in the 

 Carrizo area. The lower contact of the Tisnasbas sill with the 



