828 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
wells. Variations in thickness doubtless accompany the variations 
in material. 
The number of such strata passed in one well varies greatly. The 
drill may pierce several thousand feet of shale with little change in 
character, or a single boring may traverse half a dozen arenaceous 
strata, two or three of which may yield showings of oil or gas or both. 
These more porous strata are generally well separated, one from 
another, by the intervening compact shales. Oil may be found in 
any one of such strata or may be absent from all. It may appear in 
a lower while absent from a higher stratum, and seemingly the reverse 
may be equally true. Showings of oil or gas may occur in relatively 
dense beds while absent from porous sands near by. 
The isolation of the porous beds is well illustrated by the occur- 
rence of deep veins of water in some wells and their absence from 
neighboring wells at similar depths, or the deeper waters of one well 
may be salt and those of a near-b} 7 well fresh. 
The mutual independence of the several oil pockets is not empha- 
sized by any great diversity in the character of the oils from the differ- 
ent wells. With a few exceptions there is approximate uniformity, as 
shown by standard physical tests. Considering the striking uniform- 
ity of the great body of shales, this approximate uniformity is to be 
expected. It is too early to tell to what extent the yield of one well 
may be influenced by the pumping of adjacent wells. 
Relation to folds. — 11 will be seen from the above that the accumu- 
lation of oil is not yet seen to be related to folds in the strata. Anti- 
clinal arches of impervious stata are unnecessary to farm receptacles 
for oil and gas in so dense a rock as the Pierre shales. If such folds 
exist they do not appear al Hie surface, and they can not be recognized 
from well data until the same stratum can be identified in different 
wells. This would require a degree of precision in observation and 
reports not yet attained in this field. In the meantime the ever- 
increasing closeness of the wells is making the correlation of data 
more definite. 
While there is as yet no evidence thai deformation of strata has 
anything to do with forming receptacles for the oil, it is not yet cer- 
tain that the distribution of oil is independent of folds. According 
to the most plausible theories w r e may suppose that the substance of 
these oils was at a former time disseminated through lower rocks rich 
in organic matter. The concentration of this widely disseminated 
bituminous matter implies a movement through the rocks, perhaps 
for long distances. Such movement is conditioned by the texture of 
the rocks traversed. The permeability of rocks in the axial plane of 
an anticline may differ materially from that in the axial plane of a 
syncline. It is conceivable, therefore, that in a system of folds, how- 
ever gentle, the upward movement would be affected both in velocity 
and in direction by the position of the rocks to be traversed, whether 
