BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 32, pp. 249-266 JUNE 30, 1921 



IGNEOUS GEOLOGY OF SOUTHEASTEEN IDAHO ^ 



BY GEORGE ROGERS MANSFIELD " 



(Presented before the Society December 28, 1920) 



CONTENTS 



Page 



Introduction 24U 



General distribution and eliaracter of rocks 251 



Early rhyolitic ash 251 



Hornblende andesite porphyry 252 



Rhyolite '. 253 



Olivine basalt 255 



Contact metamorphism 258 



Epochs of igneous activity 258 



Origin of the igneous rocks 260 



Modes of eruption 261 



Generally effusive in character 261 



Intrusion at Sugarloaf Mountain 261 



Extrusion of rhyolite 264 



Extrusion of basalt 261 



Succession of igneous rocks 265 



Relations of igneous rocks to sedimentary rocks 265 



Age of igneous rocks 266 



Introduction 



The detailed examination by the U. S. Geological Survey of the Idaho 

 Phosphate Eeserve has made necessary the mapping of considerable areas 

 occupied by igneous rocks. Such rocks are abundant in southeastern 

 Idaho and present many interesting features. The igneous rocks of the 

 Fort Hall Indian Eeservation have already been described.^ In the pres- 

 ent paper it is proposed to summarize the igneous geology of the Cranes 



1 Manuscript received by the Secretary of the Society .Tuly 2, 1920. 



2 Published by permission of the Director of the IJ. S. Geological Survey. 



3 G. R. Mansfield and E. S. Larsen : Nepheline basalt in the Fort Hall Indian Reserva- 

 tion, Idaho. Washington Acad. Sci. .Tour., vol. 5, no. 1.3, .Tuly 19, 1915, pp. 4G3-468. 



G. R. Mansfield : The geography, geology, and mineral resources of the Port Hall In- 

 dian Reservation, Idaho, with a chapter on water resources by W. B, Heroy. U. S. Geol. 

 Survey BulL 713, pp. 57-61. 



(249) 



