MANSFIELD: MESOZOIC OF FORT HALL RESERVATION 



33 



TABLE I— Continued 



The Thaynes limestone has in this area been raised to the 

 rank of a group consisting of three formations, and the Nugget 

 sandstone has been subdivided into four members. The Anka- 

 reh shale, owing to change in lithologic character, becomes the 

 Ankareh sandstone. Additional interesting features were ob- 

 served in the Woodside shale. 



The purpose of this paper is to describe briefly the formations 

 which were subdivided and to explain the names used. The 

 writer is indebted to Dr. G. H. Girty for his help in joint study 

 of the formations in the field and for the determination and dis- 

 cussion of the fossils collected. 



GENERAL STRATIGRAPHY OF THE REGION 



The stratigraphic sequence in the Fort Hall Indian Reserva- 

 tion is rather full, including all the great Paleozoic and later 

 systems except the Cretaceous. There is, too, a great body of 

 igneous rocks, chiefly extrusives, in both massive and fragmental 

 form. The structure is complex, with folding and much faulting. 

 Some of the systems are poorly represented and the identifica- 

 tion of strata tentatively assigned to them uncertain. This is 

 particularly true of the Silurian. The Paleozoic formations of 

 the reservation probably agree in name and number with those of 



