70 GEOLOGY OF SW. IDAHO AND SE. OEEGON. [bull. 217. 
are commonly associated; and by the characteristics of the strata in 
which they occur. An attempt is here made to state in nontechnical 
language the conclusions that have presented themselves in reference 
to the natural storage of petroleum and gas, as search for these 
substances has been carried on in various parts of the world. No 
originality is claimed for what follows, as it is simply a rearrangement 
of widely current knowledge. 
The physical properties of petroleum, gas, and water are well 
known, and of these properties the ones of special significance to the 
geologist are their specific gravities. The specific gravity of petro- 
leum in most all instances is below 1 — that is, it is lighter than an 
equal volume of water. The range in specific gravity for American 
petroleum is from about 0.771 for the highly liquid yellow or amber- 
colored varieties to 0.945 for the thicker and frequently black mate- 
rial, valued principally as a lubricant. The only instance known, so 
far as the writer is aware, of petroleum wnich is heavier than water 
is that obtained at Zante, in Greece, which has a specific gravity of 
1.02. Natural gas is far lighter than the lightest petroleum, its den- 
sity being about one-half that of air. The principal physical differ- 
ences between petroleum and gas are those that constitute the 
distinction between a liquid and a gas. The gas has the property of 
indefinite expansion, and when put under pressure diminishes in vol- 
ume, at the same time acquiring greater expansive energy or tension. 
It is the occurrence in nature of petroleum and gas with these widely 
different physical properties, and each of them lighter than water, 
with which they are usually associated, that the geologist has to con- 
sider. Not only this, but attention needs to be given to the occur- 
rence of these substances singly or together, and of one or both of 
them in association with water. The physical properties of petroleum 
and gas thus determine some of the conditions on which success in 
the search for commercial quantities of these substances depends. 
If water, petroleum, and natural gas be placed in a corked bottle 
and the receptacle be allowed to rest for a brief time, they arrange 
themselves in the order of their respective specific gravities, the water 
being at the bottom, the petroleum next above, and the gas at the 
top. This conclusion is self-evident, and there is no reason for think- 
ing that the law it illustrates is violated when the three substances 
named are present in a cavity in the earth, or what is much more 
common, contained in the pores of a loose-textured rock. Then, too, 
the water present in the rocks is never pure, but contains more or less 
mineral matter in solution, and not infrequently it is highly saline. 
Its specific gravity is, therefore, greater than if it was pure, and its 
ability, under certain conditions, to displace petroleum and cause it 
to rise is correspondingly increased. 
The displacement by water of petroleum contained in the pores of 
rock has certain limitations, however, which at times become impor- 
tant. Petroleum in porous rocks adheres to the surfaces with which 
