METAMORPHISM IN OIL-BEARING SHALE 757 
CAUSE OF THE ALTERATION 
There can be little doubt that the burnt shale is in all cases the 
‘result of heat produced by combustion of the hydro-carbon contents 
of the shale. 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 such places is frequently impregnated with petroleum 
and the cracks partially filled with it. The altered shale areas are 
almost invariably situated in the vicinity of oil seepages, which usually 
connote 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 
rocks round about—changes in every way similar to those in locali- 
ties of the metamorphosed shale where no fire exists at present— 
gives the best of clues to the manner in which the shale has been 
baked in other cases. It is difficult to conceive of a 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. Probably 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 or gaseous hydrocarbons. 
The failure to smother the fire in the shale on Graciosa Ridge, as 
previously mentioned, indicates that such fires are able to survive 
with a limited 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. 
The cause of ignition may be kindled fires, lightning, or the spon- 
taneous combustion of the hydrocarbons or surface vegetation. 
Many of the recent cases of burning are directly traceable to 
the first cause, but for those which may have taken place before 
the advent of man, either the second or third cause will have to be 
invoked. 
Burnt shale was noticed by J. D. Whitney in the vicinity of Santa 
Barbara, and its origin rightly interpreted. He says of it: 
About five miles southeast of Carpinteria, the rock presents exactly the appear- 
ance of having had the bituminous matter burned out of it; it assumes various 
