at are devoid of interbedded dark
shales, thin-bedded dark impure limestones, dark marls, or
thin-bedded limy and fossiliferous sandstones.

(_g_) Practically all oceanic abyssal deposits; these, however, are
but rarely present on the continents.

2. Possible petroliferous areas.

(_a_) Highly folded marine and brackish water strata younger than
the Jurassic, but more especially those of Cenozoic time.

(_b_) Cambrian and Ordovician unfolded strata.

(_c_) Lake deposits formed under arid climates that cause the
waters to become saline; it appears that only in salty
waters (not over 4 per cent?) are the bituminous materials
made and preserved in the form of kerogen, the source of
petroleum; some of the Green River (Eocene) continental
deposits (the oil shales of Utah and Colorado) may be of
saline lakes.

3. Petroliferous areas.

(_a_) All marine and brackish water strata younger than the
Ordovician and but slightly warped, faulted, or folded; here
are included also the marine and brackish deposits of relic
seas like the Caspian, formed during the later Cenozoic. The
more certain oil-bearing strata are the porous thin-bedded
sandstones, limestones, and dolomites that are interbedded
with black, brown, blue, or green shales. Coal-bearing strata
of fresh-water origin are excluded. Series of strata with
disconformities may also be petroliferous, because beneath
former erosional surfaces the top strata have induced
porosity and therefore are possible reservoir rocks.

(_b_) All marine strata that are, roughly, within 100 miles of
former lands; here are more apt to occur the alternating
series of thin and thick-bedded sandstones and limestones
interbedded with shale zones.

The extent to which marine or brackish water conditions of sedimentation
are requisite to the later formation of oil, as is suggested in the
above quotation, has long been a debatable question. It may be noted
that certain oil shales formed in fresh water basins contain abundant
organic matter which is undoubtedly suitable for the generation of oil
and gas, and that these shales on distillation yield oil essentially
like that obtained from oil shales of marine orig