M. R. Thorpe— The Abajo Mountains, Utah. 385 



5. Several monoclines south and southwest of the 

 Abajo area are considered by Gregory to be of pre-Ter- 

 tiary age, while the uplift of the mountains is regarded 

 as having taken place in the Tertiary. 



The above considerations offer two lines of argument 

 in support of the theory of an uneven floor for this group. 

 The first depends upon the irregular vertical and hori- 

 zontal distribution of the laccoliths and the second 

 involves the antecedent structures, existing on all sides 

 of the area, which apparently underlie the superimposed 

 igneous masses. 



Roof. 



The roof of the Shay Mountain laccolith is lower than 

 the base of the La Plata, since this formation is involved 

 in the doming. It is impossible to determine at what 

 depth lies the horizon of intrusion for erosion has not cut 

 sufficiently deep into the uplift. Upper McElmo and 

 Dakota formations roof the southern and western sec- 

 tions of the Sierra Abajo, while the northern half was 

 covered by Mancos shale. Since the crests of the moun- 

 tains are either uncovered or still retain only isolated 

 patches of the roof rock, the former configuration of this 

 roof cannot be definitely known. From the shape of the 

 igneous mass, which does not present any marked irregu- 

 larities on its upper surface, and from dips of the sedi- 

 mentary areas on and at the base of the uplifts, it is 

 inferred that the roof was fairly regular and but slightly 

 faulted as shown by the tilting to which some of the resid- 

 ual roof blocks have been subjected. 



A considerable thickness of strata has been eroded 

 from the cover of the laccoliths and only metamorphosed 

 remnants of the roof now remain. From the data fur- 

 nished by these residual masses it is evident that Dakota 

 and McElmo strata and in all probability Mesa Verde 

 sandstone also formerly covered the laccoliths. It 

 remains to be demonstrated whether any Tertiary or later 

 sediments were ever deposited over this area, for none 

 now exist except alluvial deposits which were laid down 

 since the intrusion. The lowest strata observed in con- 

 tact with the intrusive rock are the McElmo (Jurassic), 

 while the highest formation is the Middle Mancos (Ben- 

 ton). It is not known that the Mesa Yerde (Upper Cre- 

 taceous) covered this area at the time of intrusion of the 



