756 RALPH ARNOLD AND ROBERT ANDERSON 
of the original shale is slight, the lighter varieties having a specific 
gravity less than that of water, the excessively burnt shale is very 
heavy. The material has evidently contracted to much less than its 
original volume, the angular cavities and irregular vesicles being one 
consequence of this contraction. 
Under the microscope the more advanced stages appear as having 
an exceedingly fine-grained, amorphous, porous ground-mass dis- 
colored with a reddish brown or gray stain. Black filaments and dots 
appearing like carbonaceous material are common. Exceedingly 
minute rounded and irregular grains scattered through the whole, 
but forming no appreciable proportion of it, are the only portions 
visible under crossed nicols. They extinguish four times in a revo- 
lution of the field, and are probably clastic quartz grains. ‘These are 
characteristic of the unaltered shale as well. 
Mr. George H. Eldridge remarks’ on an occurrence of burnt shale 
in the Santa Maria field near the old Blake asphalt mine south of 
Graciosa Ridge. He says, ‘‘The shale now appears red, ash-like to 
hard and clinker-like, glazed or silicified; bodies of bitumen con- 
tained within this have the appearance of a coke, as though derived 
from the solid fixed carbon of the petroleum.” 
The likeness of varieties of the burnt shale to volcanic rocks is 
indicated by the fact that Thomas Antisell in his account of the 
geology of the Coast Ranges in the Pacific Railroad Report,’ de- 
scribes “‘scoriaceous” and “‘amygdaloidal lava,” “whitish-gray, hard 
trachytic rock,” ‘“‘volcanic,” and “igneous rocks” in the region of the 
Santa Ynez River, evidently having reference to the burnt shale. 
He considered it to be in eruptive masses, forming the oldest and 
axial rocks of the hill ranges, whereas it is part of the Monterey shale 
formation which overlies the basement formations. He regarded the 
associated diatomaceous shales in some places, although not in others, 
as “‘magnesian” and “‘tremalite” rocks of igneous origin. He refers 
to the places where the shale is burning as examples of present vol- 
canic activity. 
t “The Asphalt and Bituminous Rock Deposits of the United States,” Twenty- 
second Annual Report, U. S. Geological Survey, 1901, Pt. I, p. 428. 
2 Explorations and Surveys for the Pacific Railroad, -Vol. VU, pp. 65-72. Washing- 
ton, 1857. 
