Rocks of Southern California.—Hershey. 283 
The railroad between Barstow and San Bernardino reaches 
the granite mass at mile-post sixteen,‘and at mile-post seventeen 
there are two round granite knolls standing in the center of the 
valley near Mohave river. They are of the light colored gran- 
ite with scarcely any mica or hornblende. ‘Then the granite 
border circles around by the west and is not again touched by 
the railroad until near mile-post thirty-three, beyond which for 
several miles it traverses a narrow rock gorge excavated by 
Mohave river in the solid granite rock giving splendid expos- 
ures. The mass of the rock is a medium-grained, light gray, 
~ hornblende-biotite granite or the typical grano-dioryte of the 
Sierra Nevada region, very closely resembling the Rocklin 
granite. Near mile-post thirty-three are areas of the light pink 
granite with little or no hornblende or mica, but which shows 
“a strong tendency to develop bands of a coarse pegmiatitic 
structure. It is the typical pink granite of Mohave desert. 
Here it seems to occur as dikes in the granodioryte. In the 
walls of the gorge, very narrow dikes of pink pegmatyte occur 
frequently in the gray grano-dioryte. 
The granite belt continues south into the San Bernardino 
and Sierra Madre mountains. In Cajon pass the relation be- 
tween the two varieties was placed beyond doubt. The wagon 
road. between Summit and Cajon stations enters a rock gorge 
about a mile and a half from the latter. The granite is mainly a 
gray hornblende-biotite granite or grano-dioryte, rather more 
basic than usual in southern California, but not differing from 
certain phases of the intrusive grano-dioryte of the Sierra Ne- 
vada region. Narrow dikes of light pink, coarse, pegmatitic 
granite traverse the gray granite. Finally these pink dikes 
become very abundant, enlarge and pass into a large solid 
mass or small batholith of pink granite of the kind so fre- 
quently observed on Mohave desert, removing all doubt o7 
the pink granite, even when occurring in large masses being 
intrusive in the gray grano-dioryte. 
I have concluded that the whole granite area is probably 
mainly of grano-dioryte in which the pink granite is intruded 
in small batholiths and dikes; but the latter and especially the 
pegmatyte is the more resistant and usually forms the out- 
crops. 
