76 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bull. 21/- 
The influence of an inclination of the strata and of folds has already 
been briefly considered, but the importance of structure, to use a 
general term to designate the position in which the beds occur, is so 
important in the concentration of oil and gas into productive pools 
that it needs to be well understood by all who search for these 
substances. 
In many regions the rocks are crumpled or corrugated into a series 
of up and down folds, termed, respectively, anticlines and synclines, 
as shown in fig. 1. Under the structural conditions there indicated 
it is evident that if petroleum and gas occur in the dotted stratum, 
assumed to be a porous sandstone between two thick beds of shale, 
and water is present, the petroleum will accumulate beneath the 
anticlines, and if evaporation occurs, as is the rule, gas will be 
generated, and both gas and hydraulic pressure will be present. 
These anticlinal folds, it will be remembered, have a length usually 
much greater than their width, and ma} 7 measure scores of miles' 
in their various dimensions. The longer axes of the folds may be 
horizontal or inclined, or pitch toward one end or the other. If the 
axes are horizontal it is evident that a well put down anywhere 
along the crest of a fold, as at b in the above cross section, should 
admit of a flow of gas followed by a rise of oil in the well, or its 
discharge, fountain-like, at the surface, according to the pressure. 
Should the anticlines have a pitch, however, one end of the fold being 
depressed or the other elevated, with reference to a horizontal plain, 
then it is manifest that the petroleum and gas would be concentrated 
near the upper portion of the pitching anticline and the area of the 
resulting ' ' pool " consequently lessened. Other complexities of struc- 
ture may be present, as, for example, where one series of folds crosses 
another series, thus producing conditions favoring the accumulation 
of several relatively small pools. Sometimes the rocks, instead of 
being folded into long, narrow anticlines, are elevated in certain 
regions so as to assume dome-like forms. A broad, low dome in a 
series of oil and gas charged beds, with the requisite succession of 
porous and impervious strata, would form the ideal conditions for the 
concentration of the substances named. 
The conditions briefly explained above form the basis of the well- 
known "anticlinal theory" of the occurrence of oil and gas pools, so 
ably worked out by Prof. Edward Orton in Ohio and Prof. I. C. White 
in Pennsylvania and West Virginia. 
A variation of the conditions just considered occurs when the strata 
are gently inclined over broad areas, and the rocks are bent along 
certain axes without being folded so as to form true anticlines, but 
produce flat terrace-like areas in a region of gently-inclined beds. 
Such "terraces" or " arrested anticlines" occur in Ohio and have been 
described by Professor Orton. For example, oil-bearing rocks about 
2,500 feet thick enter the Macksburg oil field, dipping gently south- 
