140 OIL FIELDS OF TEXAS-LOUISIANA COASTAL PLAIN, [bull.212. 
CONCLUSION. 
This great diversity of views regarding the origin of petroleum is 
equaled by the diversity in character of the petroleum itself and in 
the geologic conditions under which it is found. In fact, it is proba- 
ble that the final theory will include most of those outlined above, 
and will recognize the fact that this substance which is so widely dis- 
tributed in nature may be the product of widely different processes 
acting upon a great diversity of materials. Thus the hydrocarbons 
which have been observed in certain volcanic rocks and in gases 
given oif from volcanic vents may be entirety inorganic, resulting 
from the reaction between water and heated metallic carbides. The 
oil of the Appalachian field may be derived from the slow distillation 
of plant remains disseminated through the underlying shales, and 
that of the Trenton limestone of the Lima field from animal remains 
originally contained in the rocks in which it is now found. Finally, 
the oil of the Gulf Coastal Plain is probably derived, in part at least, 
from the action of decomposing organic matter, both animal and 
vegetable, but chiefly the latter, upon g}^psum. 
ACCUMULATION OF THE OIL. 
CONDITIONS FOR ACCUMULATION. 
Hydrocarbons of the petroleum type are among the most widely dis- 
tributed substances in nature. They are found associated with almost 
all classes of rocks, both crystalline and sedimentary, from the oldest 
to the youngest. While these hydrocarbons are very widely distrib- 
uted they are, however, usually in small quantities, and accumula- 
tions sufficiently large to be of commercial importance are restricted 
by certain well-defined conditions to a relatively small portion of the 
earth's surface. In many regions it is comparatively easy for the 
geologist, by an examination of surface conditions, to state definitely 
and with certainty that no oil in commercial quantities will be found. 
In other large regions he can state that oil ma} 7 be found, and can 
point out in some cases the most favorable localities, but he can not 
predict the actual occurrence of an oil pool in advance of drilling. 
The essential conditions for the accumulation of oil are (1) a suffi- 
cient supply of oil derived from any of the sources above described, 
(2) a porous reservoir rock in which it may be stored, and (3) an 
impervious cap rock which will prevent its escape. Conditions which 
favor its accumulation but are not always essential are (4) gentle 
undulations of the strata forming anticlinal arches or domes, (5) the 
complete saturation of the rocks with water and its slow circulation 
under l^drostatic head or convection due to differences of temperature. 
THE OIL SUPPLY. 
This is of course the first and most essential condition, for without 
it no accumulation could take place however favorable other condi- 
