78 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bull. 217. 
ous stratum. A well drilled in such a bed should furnish petroleum, 
but not under pressure. A small but long continued yield by pump- 
ing might be expected, the petroleum slowly percolating into the well 
from the adjacent country rock in much the same way that an ordi- 
nary surface well is supplied with water. Under the conditions just 
postulated, but petroleum as well as water being absent, a well might 
yield a moderate but long-continued supply of gas. 
Another consideration which may perhaps have economic impor- 
tance is that the presence of petroleum, especially the heavier varie- 
ties, in a porous rock retards the passage of water. The outcrop of 
a porous bed, in which heavy oil has been produced by the evapora- 
tion of a lighter variety, might seal the pores of the rock and thus 
prevent the inflow of water, and therefore the cessation of the outflow 
of petroleum. These results might come about especially in a warm 
climate and modify the conditions under which petroleum and gas 
are usually stored. The formation of solid hydrocarbons in porous 
beds and in fissures is but a further step in this same direction. 
Bodies of petroleum-bearing sandstone or other rocks above the sur- 
face drainage of a region might, on account of the formation of 
viscous or solid hydrocarbons in their superficial portions, be made 
to yield heavy oil if penetrated by horizontal wells. 
The opening of fissures communicating with the surface above an 
oil pool is considered fatal to it as a reservoir of commercial value, 
but if the oil is not under hydraulic pressure or if the pressure is not 
sufficient to cause it to overflow and be replaced by water, the petro- 
leum may not rise in the fissure, or if it does rise not reach the sur- 
face, and on evaporating, seal the breach and renew the conditions 
favorable for natural storage. Fissures filled with solid hydrocarbons 
may therefore be considered as favorable indications of the presence 
of oil pools in their vicinity. While the conditions under which petro- 
leum and its derivations occur in nature are simple and easily under- 
stood, the application of theory to practice is beset with difficulties 
and uncertainties. The information that is wanted is the texture of 
the rocks, the order of their occurrence, and their structure at a depth 
of several hundred or perhaps several thousand feet below the sur- 
face, and this, too, when the surface is usually covered with vegeta- 
tion and soil, alluvial deposits, and possibly a thick sheet of glacial 
drift. While the geologist is better able than anyone else to judge 
of the conditions to be expected within the earth's crust, it must be 
confessed that the only practicable way to secure the desired facts in 
untried regions, is in many instances, to put down test wells at what 
seem the most favorable localities. One important duty of the geolo- 
gist is to discourage ventures in regions where the rocks are so greatly 
disturbed and broken that gas or water-tight reservoirs can not exist, 
or when other conditions such as the metamorphism, preclude 
the probability of success. The conditions which have led to the 
