34 
SANTA MARTA OIL DISTRICT, CALIFORNIA. 
unaltered condition it resembles chalk, but is of siliceous instead of 
calcareous composition. 
The Monterey in the part of California treated here may be divided 
on lithologic grounds into two parts, although there seems to be per¬ 
fect conformity throughout the series. There is no definite dividing 
line to be drawn, but taken as a whole the lower half, composed 
chiefly of hard, metamorphosed, in places flinty shales, is distinct 
from the upper half, in which soft shale, giving evidence to the naked 
eye of its organic origin, is predominant. 
LOWER DIVISION. 
The fossiliferous limestone at the top of the Vaqueros is overlain 
conformably by hard calcareous and flinty unfossil if erous shale char¬ 
acteristic of the base of the Monterey. In places the limestone at 
the top of the Vaqueros is not well developed, but is replaced by a 
series of thin-bedded, in the main fairly hard, siliceous, calcareous, 
and somewhat argillaceous shales of coarse and fine texture, in which 
no well-defined line of demarcation between the two formations is to 
be drawn. The Vaqueros and Monterey terranes taken as wholes 
are distinct units, representing periods of deposition of entirely differ¬ 
ent character. As indicated by the rocks, deposition was continuous 
between the Vaqueros and Monterey and the change in character 
came suddenly, although less so in some places than in others. The 
general nature of the Vaqueros series is detrital; that of the Mon¬ 
terey is organic. The former contains many well-preserved mollus- 
can forms, the latter few. Close to the line between the two, beds 
predominatingly of a gravelly or sandy nature or those bearing fossil 
mollusks are considered part of the Vaqueros; those of a fine texture 
and of flinty or opaline or chalcedonic nature, part of the Monterey. 
Above the transitional limestone horizon between the Vaqueros 
and Monterey the lower half of the latter formation consists of a 
thick series of thin-bedded, hard, brittle, siliceous and calcareous 
shales, with local gradations on the one hand into beds of the hardest 
flint and on the other into soft diatomaceous earth. Near the base 
there is usually a horizon of black, brownish, or wax-colored flint 
in heavy beds one to several feet thick, and similar massive beds of 
peculiar sand-colored limestone with characteristic lamellar weath¬ 
ering. The greater part of the series is made up of brittle siliceous 
shale, usually much fractured and rather commonly crumpled, in 
beds averaging about one-half to 1 inch in thickness, in places alter¬ 
nating with thin shaly calcareous beds or massive strata of lime¬ 
stone. Pis. Ill, B, and VI, B (p. 46), show outcrops of typical flinty 
shale of the lower division. Beds of flinty shale or of pure flint are 
included here and there. The flint is of different colors—amber, black, 
milky, red, brown, etc.—and of different degrees of translucency. 
Much of it has been fractured and recemented with chalcedonic veins. 
