38S M. R. Thorpe— The Abajo Mountains, Utah. 



ous rocks so that they resist erosion and stand ont as 

 revetcrags, ridges and other topographic forms. The 

 effects of the metamorphism is felt from 10 to 200 feet 

 from the contact, depending npon the size of the laccolith 

 with which it is in association. The effect produced in 

 the igneous rock is apparent only a few feet at the most 

 from the contact. Megascopically, the three formations, 

 McElmo, Dakota and Mancos, have neither gained nor 

 lost any element in their mineral content other than being 

 slightly mineralized in very thin bands close to the con- 

 tact. 



Cause of the lack of intense Contact Metamorphism. 



It is apparent from the above discussion that contact 

 metamorphism has not been an important factor in the 

 geology of the Abajo Mountains. In the Carrizo area 

 Emery states that the effects of metamorphism "are 

 commonly not noticeable at a distance of more than three 

 feet from the contact, 3 and Gilbert, in the Henry Moun- 

 tains, found many instances where no difference could be 

 detected between specimens collected at the contact and 

 at the interior of the laccolith, while the original struc- 

 tures of the enveloping rocks have not been altered, 

 although they have been indurated. This phenomenon 

 is characteristic of laccoliths in the Southwest and Cross 

 attributes it to the lack of the mineralizing agents, chlo- 

 rine, fluorine and superheated steam in the magma. 4 

 The conditions of metamorphism in the Abajo area are 

 believed to be due to this cause. 



Manner of Intrusion. 



No evidences of stoping or assimiliation have been 

 found in this area, and from this and other facts above 

 mentioned, it seems conclusive that the Sierra Abajo owe 

 their origin to intrusion. The character of the horn- 

 blende-latite porphyry of the Abajo Mountains is very 

 uniform and it seems reasonable to suppose that all of the 

 laccolithic material came from a common reservoir. In 

 regions where the strata are practically undisturbed 

 except by intrusions, as in central Montana and south- 

 eastern Utah, the magma must have been aggressive, 



8 Emery, W. B.: this Journal, 4th Ser., vol. 42, p. 363, Oct., 1916. 

 4 Cross, Whitman, Spencer, A. C, Purrington, C. W. : U. S. Geol. Surv., 

 La Plata folio, No. 60, Geol. Atlas of U. S., p. 11, 1899. 



