72 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bull. 217. 
The impervious rocks met with in stratified beds, which are the 
usual ones containing water, petroleum, and gas, are mainly shale 
and clay. These rocks were once in the condition of mud and silt, 
and are composed of very small fragments, and, moreover, on account 
of their consistency, are rendered compact by pressure and are fre- 
quently impervious even to gases. 
The porosity of rocks thus depends on the size and shape of the 
grain, or crystals of which they are composed, and on the nature and 
amount of the cement which unites the grains; the smaller the grains 
and the more perfectly they are cemented together, the less the size of 
the interspaces between them. Pressure tends to compact the gran- 
ules of rocks, and the smaller the granules the more thoroughly they 
may be made to fit together. The search for petroleum and gas, 
therefore, demands a studj^ of the rocks in reference to their porositj^. 
As will be shown below, the most favorable conditions, so far as rock 
texture is concerned, occur when a thick layer of open-textured rock, 
say coarse sandstone, is covered by a layer of fine-grained rocks like 
shale. An additional reason why sandstone and certain limestones 
make favorable reservoir rocks, and shales efficient covers for the 
reservoirs, is because the sandstones and limestones are rigid, and if 
fractured or jointed the cracks remain open unless the beds are under 
excessive pressure, while the shales, if fractured, tend to flow, even 
under moderate pressure, and thus close the fissures. 
The source of the water with which nearly all rocks near the earth's 
surface are more or less thoroughly charged, is primarily the rain, 
although essentially all stratified beds were deposited in water and 
for that reason were at first water charged, but this primary water 
has in most instances been flooded out by subsequent underground 
percolation. 
The source of the petroleum, as conceded, I believe, by all geologists, 
is organic matter contained in the rocks, but other theories to account 
for its presence have been suggested. The chemistry of the change 
undergone by organic matter when excluded from the air is not well 
known, but the evidence seems conclusive that plant or animal 
remains buried in the rocks, usually in the presence of saline water, 
undergo a slow spontaneous change at low temperatures, one result of 
which is the production of petroleum. A higher degree of heat 
hastens the process, but within moderate limits seemingly does not 
vary the nature of the result. 
Petroleum at ordinary surface temperature evaporates and gives 
origin to inflammable gases, and an increase in temperature hastens 
the change, but, except at what may be termed a high degree of heat, 
does not vary the nature of the result. What is of special interest to 
the geologist in this connection is that the evaporation of petroleum 
and the production of gas go on at the ordinary temperatures observed 
in the surficial portions of the earth's crust. As light, highly liquid 
