382 if. R. Thorpe — The Aba jo Mountains, Utah. 



Mancos shale. Only on the east side of Horsehead Peak 

 does the Dakota overlap the igneous rock. Fossils found 

 in metamorphosed shale near the summit of North Peak, 

 11,350 feet high, were identified by Dr. Stanton as Middle 

 Mancos. Isolated tilted blocks of Mancos shale lie at 

 different altitudes, dipping in various directions and at 

 different angles. At the mouth of North Canyon the 

 Dakota and porphyry are in contact, the former dipping 

 80° northwest at the contact, decreasing to 16° E. about 

 200 feet from this point, and then sloping away for sev- 



Fig. 3. 



Fig. 3. Eastern slope of Mount Linnaeus. 



eral miles, gradually flattening out, and merging into the 

 regional dip of the plain. The Dakota sandstone is 

 apparently the floor of the laccolith except on the eastern 

 side where the magma, instead of breaking through, 

 arched up the sandstone as in the case of the southern 

 half of the group. 



Between Johnson and Eecapture Creeks a large sheet 

 of dacite outcrops. It is an offshoot of the magma form- 

 ing the major intrusions. Its roof has been very nearly 

 removed by erosion, but many isolated patches of Dakota 

 sandstone as well as some strata of the McElmo still 

 remain. 



