GEOLOGY OF THE SOUTHERN COAST RANGES. 569 
eral of the northern ones, the andesite to those nearer San Luis 
Obispo. The important constituents of the former are quartz, 
biotite, and an acid plagioclase feldspar ; of the latter labrador- 
ite, biotite, and enstatite, as porphyritic constituents in a granular 
base. Morro Rock is the most striking topographic feature on 
the coast of California. 
The diabase intrusive in the Knoxville beds has a character 
different from any other rocks in the region under investigation. 
It is grayish in color, fine grained, and amygdaloidal upon the 
edges. It has come up in great dikes or sheets on either side 
of the main Knoxville area, throwing the latter into a synclinal 
trough. It extends northwest from Cuesta Pass for many miles, 
and is quite uniform save for local gabbroitic variations. 
The peridotites and related feldspathic rocks of the Coast 
Ranges are well known. The great body of this rock in the 
San Luis Obispo region has been changed to serpentine. It con- 
sisted originally of olivine and augite. Local variations rich in 
feldspar occur along the Santa Lucia range forming diabase and 
gabbro. The feldspathic facies is quite extensively developed 
not only upon the borders of the great serpentine area north of 
San Luis Opispo, but also in many places as apparently inde- 
pendent intrusions. From their similarity in character it is easy 
to see, however, that all these rocks are genetically related. 
The serpentine has the usual sheet or lense-like character con- 
formable to the dip and strike of the inclosing rocks, which 
usually stand very steeply. 
The igneous rocks of Tertiary age have a great range in 
chemical and mineralogical composition. The succession of 
these rocks was only partly determined. The oldest is the 
rhyolite of which there are two varieties. One contains free 
quarts and occurs as a very limited surface flow, being mostly 
represented by tuffs. The other has the form of long, narrow 
sheets intruded near the base of the Miocene, and contains no 
free quartz, but an abundance of plagioclase phenocrysts. The 
ash which is so widespread at this horizon appears to be con- 
nected with rhyolite eruptions, but these particular sheets are 
