CHAPTER II. 
GEOLOGICAL RELATIONSHIP OF THE ROCKS OF THE MONTEREY 
DISTRICT. 
GENERAL DESCRIPTION OF THE AREA STUDIED. 
This bulletin will treat chiefly of the geology of that portion of South 
Mountain which lies in Franklin and Adams counties and is embraced 
between the lower Cambrian quartzites of the Green Ridge on the 
west and the Triassic red sandstones of the plains on the east. The 
southern limit is the State line, while a parallel line passing through 
the Russel copper mine, some 6 miles to the north, furnishes a northern 
boundary. 1 (See PL I.) Well- wooded mountains (Green Ridge, Mou 
terey Peak, Pine Mountain, Jacks Mountain, Raven Rock Mountain, 
Haycock Mountain, etc.), varying in altitude from 1,500 or 1,850 feet, 
with a general trend from northeast to southwest, inclosing high 
plateau-like valleys and sloping toward the eastern plains, constitute 
a region of exceptional natural beauty and geological interest. (See 
PI. II.) 
Though but a limited portion of an extended South Mountain area, 
this district is in many respects quite typical and furnishes ample mate- 
rial for some general conclusions in regard to the geology of the entire 
area. Although, because of the short time at the disposal of the writer, 
detailed field work was limited to the Monterey district, the petro- 
graphical study included material collected throughout the South 
Mountain. 
EXTENT AND CHARACTER OF THE THREE ROCK TYPES. 
Three distinct rock types are readily recognized. (PI. III.) 
1. A siliceous sedimentary rock, represented by aquartzose conglom- 
erate, a sandstone, and a quartzite. This is rarely accompanied by an 
interbedded argillaceous slate. 
2. An acid volcanic rock, which shows all phases of crystallization, 
from a spherulitic rhyolite to a true quartz -porixhyry, is amygdaloidal 
or compact, is accompanied by pyroclastics and breccias, and is some- 
times sheared into a perfectly fissile slate or sericite- schist. 
3. A basic, noncrystalline, volcanic rock, which is usually amygda- 
loidal, massive, or more frequently schistose, and is also accompanied 
by j)yroclastics and breccias and sheared to a slate. 
1 As a matter of convenience this area will be calle4 the Monterey district, 
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