276 SCIENCE IN SHORT CHAPTERS. 



ranean wall of compact rock impermeable to water, abuts a 

 great face of down-sloping strata of porous sandstone and 

 porous shales. These porous rocks receive the rain which 

 falls on the slopes of the Hope Mountain and other hills 

 which they form; this water sinks into the millstone grit 

 of these hills and percolates downwards until it reaches the 

 limestone barrier, into which it cannot penetrate. 



It here accumulates as a subterranean reservoir which 

 finds an outlet at a convenient natural fissure, and, as the 

 percolation is continuous, the spring is a constant one. 

 Some of the water travels many miles underground before 

 it thus escapes. Hundreds of other smaller instances might 

 be quoted, the above being the common history of springs 

 which start up whenever the underground waters that flow 

 through porous rocks or soil meet with compact rocks or 

 impermeable clay, and thus, being able to proceed no fur- 

 ther downwards, accumulate and produce an overflow which 

 we call a "spring." 



If water can thus travel underground, why not oil? 



Although the oil springs or oil wells are not immediately 

 above or below coal seams, they are all within " measura- 

 ble distance" of great coal formations the oil territory of 

 Pennsylvania is, in fact, surrounded by coal, some of it an- 

 thracite, which is really a coke, such as would be produced 

 if we artificially distilled the hydrocarbons from coal, and 

 then compressed the residue, as the anthracite has certainly 

 been pressed by the strata resting upon it. 



The rocks in immediate contact and proximity to coal 

 seams " the coal measures," as they are called are mostly 

 porous, some of them very porous, and thus if at any period 

 of the earth's long history a seam of coal became heated, as 

 we know so many strata are, and have been heated, a 

 mineral oil would certainly be formed, would first permeate 

 the porous rocks as vapor, then be condensed and 'make its 

 way through them, following their "dip" or inclination 

 until it reached a barrier such as the limestone forms. 



It would thus in after-ages be found, not among the coal 

 where it was formed, but at the limestone or other imper- 

 meable rock by whicn its further percolation was arrested. 



This is just where it actually is found. 



