Gold and Otiier Minerals in lozi'a. — Calvin. 369 
ter which by the slow distillation always going on must yield 
annually considerable volumes of gaseous or liquid hydrocar- 
bons. At all the exposures of the lower Trenton, from Du- 
buque northward, the dry shaly partings between the ledges of 
limestone afford material so rich in bitumen that it is easily 
lighted with a match ; it bums freely and emits a strong oily 
odor. Bituminous shale, precisely like that seen in the natural 
exposures, was brought up from the horizon of the Trenton 
in the deep well at Washington. Iowa : it has been recognized 
in other deep wells ; the same shales, rich in bitumen, probably 
underlies the greater part of the state. 
If then a great amount of bitumen is stored up in the Tren- 
ton limestone and is constantly evolving gas and oil by slow 
distillation, why are not gas and oil wells as common in Iowa 
as in the productive regions of Ohio and Indiana? Let it be 
answered that something more than petroleum-bearing rock- 
is needed in order that oil may be obtained in quantities of 
commercial importance. It has been estimated by professor 
Orton that the rocks beneath the surface over a very large part 
of Ohio contain at least 3.000.000 barrels of oil to the square 
mile, and yet not one gallon of this can be secured by the drill 
without the concurrence of at least two other conditions : ( i ) 
There must be a porous reservoir — sandstone or porous lime- 
stone — in which the oil or gas may accumulate, and this must 
be covered with shale or other impervious deposit to prevent 
the hydrocarbons from escaping to the surface and liecoming 
lost as fast as they are generated. But reservoir and cover 
alone will not insure a supply. So long as the rocks lie flat or 
have a uniform dip there will be no accumulations of any im- 
portance. (2) The reservoir and cover must present a series 
of folds beneath the arches of which the oil and gas are en- 
trapped and accumulated under high pressure. Three condi- 
tions, therefore, must exist conjointly — the source of supply 
in some form of organic matter, the porous reservoir and im- 
pervious cover, and the arched or folded condition of the beds. 
It is the last of these conditions that is wanting in Iowa. Our 
stratified rocks are not folded to anv noteworthy extent. The 
compression and crushing which gave rise to the Appalachian 
mountains produced folds as far west as Indiana, and then the 
effects fade out. Iowa is too far awav from other centers of 
