MANSFIELD: MESOZOIC OF FORT HALL RESERVATION 



31 



calculated either from direct observations on the rotation of the 

 normal solution for monochromatic light, or by the method of 

 Schonrock, in which, however, the conversion factor 34?620 is 

 used, lies in essential agreement with the previously accepted 

 value. This is concluded to be corroborative evidence of the 

 correctness of the new value 34? 620 of the conversion factor. 



GEOLOGY. — Subdivisions of the Thaynes limestone and Nugget 

 sandstone, Mesozoic, in the Fort Hall Indian Reservation, 

 Idaho. 1 G.R. Mansfield, Geological Survey. (Communi- 

 cated by F. L. Ransome.) 



INTRODUCTION AND SUMMARY 



In the field season of 1913 the writer with a U. S. Geological 

 Survey party made an examination of the Fort Hall Indian 

 Reservation in southeastern Idaho. 

 The reservation, which has an irregu- 

 lar shape and includes approximately 

 800 square miles, lies mostly between 

 the meridans 112° and 112° 45' W. 

 and between the parallels 42° 30' and 

 43° 15' N. (See fig. 1.) 



Although the main purpose of the 

 work was a mineral classification of 

 the land, considerable attention had 

 to be paid to the stratigraphy of the 

 region, and it was found necessary to 

 map some of the formations in detail. 

 This made it desirable to subdivide 

 certain Mesozoic formations, particu- 

 larly the Thaynes limestone, Lower 

 Triassic, and the Nugget sandstone, 

 Jurassic or Triassic. The strata in- 

 volved, together with the intervening Ankareh sandstone, have 

 a thickness of about 6,800 feet. 



Fig. 1. Index map of Idaho, 

 showing the location of the 

 Fort Hall Indian Reservation. 



1 Published by permission of the Director of the United States Geological 

 Survey. 



