324 G. K. GILBERT — GRAVITATIONAL ASSEMBLAGE IN GRANITE 



and subsequent deposition. When the phenomena of the boulder (which 

 happened to be first observed) stood alone, I entertained as an alternative 

 the hypothesis that the unconformity was occasioned by the fortuitous 

 juxtaposition of parts of a dislocated body of banded rock; but the un- 

 conformities of the Cooper Meadow locality do not admit of that expla- 

 nation. In each example the bands of the body below the plane of un- 

 conformity are obliquely transected, while the bands of the body above 

 the unconformity are continuous. 



In each unconformity the lowest band of the upper series, that which 

 rests directly on the eroded surface of the lower series, is one of the dark 

 bands. This fact, taken in connection with the fact that the dark bands 



are more sharply separated from 



v vU-.' :.-•.■•?■ v the pale bands below them than 



from the pale bands above them, 



suggests the hypothesis that a pair 



ws ^^ of bands — dark below and pale 



7 TT '"■'* ""'*' above^constitute the unit of de- 



Figurb 1. — Banding and Unconformity in 



Granite. position. 



As to the general nature of the deposition, two ideas have occurred to 

 me: (1) that the granite is metamorphic and the dark and pale bands 

 were originally aqueous sediment; (2) that the granite is igneous and the 

 bands were deposited from and partly eroded by liquid magma in motion. 

 The first of these is opposed by the absence of schistosity, by the fact that 

 the bands seem to lie in their original positions without distortion, and 

 by the fact that the less siliceous bands, instead of the more siliceous, lie 

 next to the planes of unconformity, thus reversing the normal order for 

 aqueous deposition. The second suggestion, of deposition from a liquid 

 magma, is too little developed for critical consideration. To constitute 

 a useful working hypothesis it should be supplemented by the suggestion 

 of conditions determining deposition and erosion. 



If deposition was from a magma, and if the unit of deposition was a 

 double layer, with dominance in its lower part of the heavy minerals, 

 mica and hornblende, and dominance in its upper part of the rela- 

 tively light minerals, quartz and feldspar, then gravity may have played 

 a role in the process of deposition. 



Inclusions 



Some of the Sierra granites are practically devoid of inclusions. Others 

 show inclusions at all exposures. A body of light gray granite in the 

 Kings Kiver country, occupying a territory of unknown extent but not 



