238 proceedings: geological society 



taries, Weminuche Creek and First Fork, a distance of 7 miles. The 

 Hayden map represents the canyon as cut entirely in Upper Dakota. 



The fall section of Paleozoic and Mesozoic formations known in the 

 San Juan region extends from the Animas Valley to the ridge east of 

 Pine River, but somewhere within a distance of 6 or 7 miles southwest 

 of Graham Peak (the point of elevation 12,336 on Hayden map) the 

 La Plata sandstone overlaps all formations from the Triassic to the 

 pre-Cambrian. This must be in the zone where the Hayden map 

 represents the ''Upper Carboniferous" in contact with the pre-Cambrian. 

 But inasmuch as a great fault-fold occurs about on the line where that 

 map represents "Upper Dakota" in contact with pre-Cambrian, west 

 of Weminuche Creek, it is possible that the unconformity below the 

 La Plata may not be exposed. 



The Hayden geologists believed that a land area of pre-Cambrian 

 rocks existed in the San Juan region during Cretaceous time. The 

 observations here recorded, with others of recent years, show, rather, 

 that this land mass was produced during the pre-La Plata interval 

 of uplift and erosion and that sediments now supposed to be Jurassic 

 were probably deposited over the San Juan area and far to the north. 

 It seems to us not unlikely that the Morrison beds on the east were con- 

 nected originally with the Gunnison on the west. 



Review of "Principles of Stratigraphy" by A. W. Grahau: G. S. Rogers. 

 No abstract. 



Clarence N. Fenner, 

 Frank L. Hess, 



Secretaries. 



