738 WALTER HARVEY WEED 
The rocks of this batholith present a wide variation in min- 
eral and chemical composition, but a study of the field relations 
shows they must be regarded not only as facies of the same 
magma, but as parts of one mass. The very basic rocks all occur 
at the margins, yet there are variations within the main body 
itself which are clearly recognizable rock types, yet cannot be 
discriminated in mapping. This difficulty has been experienced 
by those geologists working in the Sierra Nevada, where, as 
stated by Turner,’ a considerable variety of rock-types have been 
mapped as granodiorite, ‘‘although, as a rule, gabbro, even when 
genetically related to granodiorite proper, has been separated.’ 
Where detailed mapping upon a large scale map is not possible, 
this difficulty of separating parts of a single intrusive body in 
which the rock-types grade into one another can only be met by an 
arbitrary use of the name of the prevailing rock-type for the entire 
mass,as has commonly been done heretofore, or by using a generic 
term like granolite* to embrace all coarsely granular rocks. 
The prevailing rock of the batholith is a granite whose com- 
position is that given under the number 518. It is a normal 
hornblende-granite which is very generally sheeted, forming 
picturesque crags and bowlder groupings. It disintegrates readily 
into platy masses or shells which separate from the bowlders, 
and themselves crumble to a coarse sand. Over large tracts 
disintegration has reduced the rock to a smoothly rolling sur- 
face, on which scattered bowlders rise above the general level. 
Perfectly fresh material can, therefore, be obtained only where 
the rock has been quarried or exposed by mining operations. It 
is a medium to coarse-grained rock, the average size of the grains 
being 3-5mm. The grayish quartz and white feldspar grains are 
of about equal size. Black mica and dark green hornblende are 
present in considerable quantity. Under the microscope it shows 
the normal characters of a granite, but contains an unusual 
amount of plagioclase. 
tH. W. TurNER: Granitic rocks of Sierra Nevada, Jour. GEOoL., Vol. VII, 
1899, p. 146. f 
2See DURNER > lociicit., ps 141. 
