326 G. K. GILBERT GRAVITATIONAL ASSEMBLAGE IN GRANITE 



derivation from an older plutonic body. This hypothesis still seems to 

 me the more available, but is held lightly, partly because it does not read- 

 ily explain the small size of the inclusions and partly because it has been 

 compared with macroscopic data only. Using it as a working hypothesis 

 in the field, I interpreted the subangular forms of the bodies as fracture 

 forms modified by partial solution in the enveloping magma. 



Assuming the inclusions to be of fragmental origin, it seems evident 

 that they experienced partial refusion while in the including magma. 

 A plastic condition is implied by their deformation through interference 

 where they are crowded close together, and also by the fact that they 

 yielded to squeezing with the same facility as the surrounding magma. 

 Had they been more rigid than their matrix, they would have been forced 

 into contact before suffering elongation (see plate 46). 



Assuming them to be fragmental, it is an open question whether their 

 close aggregations are best explained as features of original distribution 

 or as the result of gravitational assemblage. The first explanation ac- 

 counts best for the long belts of closely grouped inclusions separating 

 tracts in which they are sparse. The second accords best with r,he 

 mingling of inclusions of diverse texture and also with the rounding of 

 angles. A mass of angular fragments associated with little more matrix 

 than was required to fill the interstices would have small opportunity for 

 surface modification by solution. 



In the vicinity of Cooper meadow, on the upper Yuba river, a very 

 different granite, a pale variety with large feldspar phenocrysts, contains 

 an abundance of small inclusions, and these also are in places closely as- 

 sembled. In this case the inclusions vary through a much wider petro- 

 graphic range and their history is more complex. 



