322 G. K. GILBERT — GRAVITATIONAL ASSEMBLAGE IN GRANITE 



bodies. The publication does not cover all my observations, but selects 

 those in reference to certain assemblages of crystals and inclusions. The 

 word "granite" is used in its broadest sense, including rocks to which the 

 discriminating petrographer would give several different names. 



Feldspar 



One of the broadly developed granite types of the Sierra is of pale 

 color, being composed chiefly of feldspar and quartz, with moderate 

 amounts of mica and hornblende. It is characterized by very large 

 phenocrysts of feldspar, the crystals ranging in diameter from one inch 

 to four inches. Ordinarily these are scattered through the rock at 

 intervals ranging from two to four or five diameters of the phenocryst, 

 but there are many spots where they are so closely aggregated as to be In 

 actual contact. Such aggregations are usually from a few feet to a few 

 yards in extent. Their boundaries may be definite or indefinite. They 

 are more abundant in regions where the phenocrysts are comparatively 

 large. The one represented in the illustration is composed of crystals 

 from 2Y2 to 4 inches in greatest diameter. The crystals of an aggregation, 

 although in contact, do not interfere one with another. Their inter- 

 stices are occupied by smaller crystals, like those of the general mass of 

 granite. These characteristics seem to me to indicate that the crystals 

 were not formed in juxtaposition, but were in some way assembled after 

 completion; and the hypothesis I suggest is that they were assembled by 

 gravitjr, being either lighter or heavier than the magma from which they 

 had crystallized. Their great size is favorable to the hypothesis that they 

 were propelled through the magma, for the propelling force, differential 

 weight, is proportional to the cube of the diameter, while the resistance 

 of the magma is proportional to the square of the diameter. 



Localities at which the phenomena have been observed are the uplands 

 about Tuolumne meadows, and the vicinity of Cooper meadow on the 

 headwaters of Yuba river. The locality of the illustration, plate 43, is 

 one mile and a half east by south of McGee lake, in latitude 37° 53'. 8, 

 longitude 119° 24'.3. 



Hornblende 



Granites of light gray color, but somewhat darker than the last men- 

 tioned, exhibited in places a similar assemblage of hornblende. The 

 hornblende crystals range in length from three-eighths to three- fourths 

 of an inch. The largest assemblages seen are 6 or 7 yards wide and their 

 limits are indefinite. In one instance there is a definite limit on one side. 

 The hornblende crystals are not so closely packed as are the feldspar 



