geology. 
51 
The likeness of varieties of the burnt shale to volcanic rocks is indi¬ 
cated by the fact that Thomas Antisell, in his account of the geology 
of the Coast Ranges in the Pacific Railroad reports,® describes “scori- 
aceous” and “ amygdaloidal lava/’ “whitish-gray, hard trachytic 
rock,” “volcanic,” and “igneous rocks” in the region of Santa Ynez 
River, evidently having reference to the burnt shale. He considered 
these rocks to be eruptive masses, forming the oldest and axial rocks 
of the hill ranges, whereas they are part of the Monterey shale, which 
overlies the basement formation. He regarded the associated diato- 
maceous shales in some places, although not in others, as “magnesian” 
and “tremolite” rocks of igneous origin, and refers to the places 
where the shale is burning as examples of present volcanic activity. 
CAUSE OF THE ALTERATION. 
There can be little doubt that the burnt condition of the shale is 
in all places the result of heat produced by combustion of its hydro¬ 
carbon content. The phenomenon is confined to the Monterey shale, 
which is the source of a large part of the California petroleum, and 
to those regions in which this formation is extremely bituminous. 
The shale in many such places is impregnated with petroleum and 
the cracks partially filled with it. The areas of altered shale are 
almost invariably situated in the vicinity of oil seepages, which 
usually denote a fractured condition of the rocks such as would 
allow fire to spread and be supported. The observance of fires 
actually in progress in the shale and the changes that have taken 
place in the neighboring rocks—changes in every way similar to 
those in localities where no fire exists at present—give the best clues 
to the manner in which the shale has been baked in other places. It 
is difficult to conceive another source of heat sufficient to cause local 
baking of the shale in otherwise unaltered strata at a depth of 1,000 
feet below the surface in such a case as has been mentioned. Prob¬ 
ably there, as on the surface, it was due to ignition of bituminous 
material. It is probable that fire started in the petroliferous shale 
at the surface and threaded its way downward along cracks partially 
filled with bitumen. The failure to smother the fire in the shale on 
Graciosa Ridge, previously noted, indicates that such fires are able 
to survive with a small air supply. On the other hand, if the above 
theory is correct, it indicates that a considerable amount of oxygen 
may be present in the rocks at such a depth. 
In this connection it may be mentioned that the temperature in a 
well near the one in which the burnt shale was found was 152° F., 
at a depth of 3,600 feet. The cause of ignition may be kindled fires, 
lightning, or the spontaneous combustion of the hydrocarbons or 
a Explorations and surveys for the Pacific Railroad, vol. 7, 1857, pp. 65-72. 
