50 
SANTA MARIA OIL DISTRICT, CALIFORNIA. 
to 1,040 feet below the surface in Hill well No. 1, in the Lompoc field, 
which are identical in appearance and texture with the burnt shale 
elsewhere. Traces of petroleum were associated with the upper 
stratum of burnt shale in this well. In numerous other wells in the 
Santa Maria field red shale, doubtless burnt, was found at depths 
ranging between 90 and 330 feet below the surface. The hardening 
consequent on the burning has in some places rendered the rock dif¬ 
ficult to pierce with the drill. 
LITHOLOGIC CHARACTER OP BURNT SHALE. 
The burnt shale exhibits all stages of change from a slight indura¬ 
tion and discoloration, due, probably, to oxidation of iron, to an 
extreme hardening and partial fusion. Where slightly altered, the 
normal white shale assumes a light-pink color. From this stage it 
passes through various shades of rose and brick-red and deepens in 
color to a reddish, bluish, or greenish black, or even a true black. In 
the advanced stages of change it becomes a rough, brittle, reddish, 
porous slag, like vesicular lava, or a very hard, compact, dark, and 
dull-colored rock, looking something like a compact igneous rock. 
An example of partly vesicular and partly compact burnt shale is 
shown in PI. V, B (p. 36). Burnt shale is not crystalline, but the 
texture is so variable as to give a patchy appearance to surfaces. In 
one place it may be compact and black, nearly full of irregular cavi¬ 
ties, surrounded by patches of different colors; in another, vesicular 
and reddish. Whereas the weight 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 rock in the more advanced stages of alter¬ 
ation appears to have an exceedingly fine grained, amorphous, porous 
groundmass, discolored with reddish-brown or gray stains. 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 revolution of the field and are probably clastic quartz grains. 
These are characteristic of the unaltered shale as well. 
G. H. Eldridge a notes an occurrence of burnt shale near the old 
Blake asphalt mine, south of Graciosa Ridge. He says: “The shale 
now appears red, ashlike to hard and clinker-like, glazed, or silicified; 
bodies of bitumen contained within this have the appearance of a 
coke, as though derived from the solid fixed carbon of the petroleum.” 
a The asphalt and bituminous rock deposits of the United States: Twenty-second Ann. Rept. 
U. S. Geol. Survey, pt. 1, 1901, p. 428. 
