BULLETIN OF THE GEOLOGICAL SOCIETY Of AMERICA 

 VOL. 17, PP. 321-328, PLS. 43-46 JULY 27, 1906 



GRAVITATIONAL ASSEMBLAGE IN GRANITE* 



BY G. K. GILBERT 



{Bead before the Cordilleran Section of the Society December SO, 1905) 



CONTENTS 



Page 



Introduction 321 



Feldspar 322 



Hornblende 322 



Banding . 323 



Inclusions 324 



Explanation of plates ' 327 



IXTRODUCTION 



In the higher parts of the Sierra Nevada the dominant rock is granite. 

 By reason of Pleistocene glaciation the exposures are exceptionally fine. 

 Over broad areas glacial erosion has removed the products of decay, laying 

 bare the unaltered rock, and large portions of these areas are free from 

 glacial debris. On most of the drift-free surfaces postglacial decay has 

 made little progress and vegetation has as yet no foothold. In many 

 places one can walk for miles on firm granite, and tracts of ideally perfect 

 exposure are often many acres in extent. Taking account of the further 

 fact that the summer climate is usually dry, I regard the region as one of 

 the finest in the world for the study of problems associated with large 

 bodies of granite. 



My acquaintance with the Sierra granites is superficial and fragmentary. 

 While engaged in physiographic and glacial studies I have traversed them 

 on several lines, and finding my attention attracted by some of their con- 

 spicuous features have made a desultory record with notebook and camera. 

 As I am not versed in either the methods or the lore of the petrographer, 

 it has not seemed best that I attempt either to round out my field observa- 

 tions or to supplement them by office study, and this publication is under- 

 taken chiefly for the purpose of directing attention to what I regard as a 

 superb field for the study of the mechanics and physics of large plutonic 



♦Published by permission of the Director of the U. S. Geological Survey. 



XXX— Bfli^ Gkol. Soc. Am., Vol. 17, 1905 (321) 



