386 M. R. Thorpe— The Abajo Mountains, Utah. 



magma, so that in estimating the thickness of the cover, 

 the top of the Mancos shale will be used as the datum 

 plane. Hence the depth of the cover over the Shay 

 Mountain laccolith must have been not less than 2,750 + 

 feet, while the depth over the northern area of the Abajo 

 group was not less than 1,200 + feet and over the 

 southern and western sections about 1,500 feet. In the 

 Carrizo area Emery estimated a depth of between 2,000 

 and 5,000 feet for the depth of the cover over that moun- 

 tain. Gilbert, in restoring the roofs of the laccoliths 

 which comprise the Henry Mountains, found a depth 

 of about 3,500 feet for the laccoliths of the upper zone 

 and for those of the lower zone nearly 7,000 feet. Gilbert 

 considered that all of the Cretaceous beds covered the 

 laccoliths of the Henry Mountains. In all probability 

 they also covered the Abajo group and in that case from 

 800 to 1,000 feet of strata must be added to the writer's 

 estimate of the depth of the cover over that region. 



Summary of proof of laccolithic origin for the Abajo group. 



Criteria in support of the theory of laccolithic origin 

 for the Abajo group have been carefully studied in the 

 field and in the laboratory and the results will be briefly 

 presented below: 



A. Factors in the igneous mass of the Abajo group which tend to prove an 



intrusive origin. 



1. Porphyritic texture is most common, indicating 

 that the rock cooled at considerable depth to allow the 

 phenocrysts to form in place, to assume their crystal 

 forms and to attain considerable size. 2. Specimens of 

 rock gathered at various horizons in the laccoliths and at 

 different localities show a remarkable similarity both in 

 composition and structure. 3. There is an absence of all 

 extrusive characteristics, such as amygdaloidal and vesi- 

 cular structures. 4. Laccoliths of this area are in asso- 

 ciation with dikes, sheets and other igneous bodies which 

 have a crosscutting relationship to the sediments. The 

 origin of these phenomena is clearly intrusive. Some of 

 the dikes cut through the cover of the laccolith and hence 

 the igneous mass must have been intruded. 



B. Factors in the sediments associated with the igneous rock which point to 



an intrusive origin for the latter. 



1. At every locality where the igneous mass is in visible 

 contact with the sedimentary strata, the latter are meta- 



