562 H. W. FAIRBANKS 
The thickness of these beds is remarkable, being 6000 to 8000 
feet. They occupy the same position with reference to the bitu- 
minous shales as similar beds on the upper San Antonio River. 
The volcanic ash forms a fairly constant horizon over nearly 
the whole of the region mapped. Several different centers of 
volcanic action seem to have existed in this region shortly after 
the beginning of the Miocene. In the mountains south of San 
Luis Obispo the ash has a thickness of fully 800 feet, while 
beginning near the Lion rock at the western end of the San 
Luis range this bed of volcanic ash extends easterly along the 
southern slope for a distance of over thirty miles. Near the 
centers of eruption the fragments are coarse, but farther away 
the deposit consists of frothy pumice, and in places occurring in 
beds of glass as fine as dust. On Old creek a small flow of 
banded rhyolite is associated with the fragmental material. It 
seems probable from the large amount of ash and small amount 
of massive lava that the eruptions were of an explosive nature 
and took place at or beneath the surface of the sea. This is the 
same eruption as that indicated at Point Sal. 
The bituminous shales and flints form the uppermost mem- 
ber of the Monterey series. They constitute the main portion of 
the San Luis range and that portion of the Santa Lucia range 
lying east of Cuesta Pass. They also underlie the younger for- 
mations in the upper Salinas Valley. This formation measured 
in both ranges appears to reach the enormous thickness of 5000 
feet. It consists almost exclusively of a thin banded silicious 
shale which over extensive areas has been changed to a flint. 
Much remains to be done in a detailed study of these shales, 
although it has been quite certainly proved that they are largely 
of organic origin.2 The shales are generally more or less 
impregnated with bituminous matter, and it appears to be rea- 
sonably certain that they constitute the main source of the oil 
and asphaltum which is so widely distributed through the Coast 
Ranges. Springs of thick oil and sulphurous water issue from 
‘Bull. Dept. of Geol., University of California, Vol. II, p. 16. 
? Bull. Dept. of Geol., University of California, Vol. II, p. 13. 
