272 SCIENCE IN SHORT CHAPTERS. 



several water-mills. It is certain that all this water is not generated 

 either by the limestone or the sandstone from which it issues, nor 

 can it be all " generated" on the spot. The true explanation of its 

 origin is simple enough. 



The mountain limestone underlies the coal-measures and crops up 

 obliquely at Holy well ; against this oblique subterranean 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 downward until it reaches the lime- 

 stone barrier, into which it cannot penetrate. 



It here accumulates as a subterranean reservoir which finds an out- 

 let at a convenient natural fissure, and, as the percolation is contin- 

 uous, the spring is a constant one. Some of the water travels many 

 miles underground before it thus escapes. Hundreds of other simi- 

 lar 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 further downward, accumu- 

 late 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 " measurable distance" of great 

 coal formations the oil territory of Pennsylvania is, in fact, sur- 

 rounded by coal, some of it anthracite, 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, v/ould 

 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 impermeable rock by which 

 its further percolation was arrested. 



This is just where it actually is found. 



Limestone, although not porous like shales and sandstones, is 

 specially well adapted for storing large subterranean accumulations, 

 on account of the great cavities to which it is liable. Nearly all the 

 caverns in this country, in Ireland where they abound, in America, 

 and other parts of the world are in limestone rocks ; they are espe- 

 cially abundant in the " carboniferous limestone" which underlies the 

 coal-measures, and this is explained by the fact that limestone may 

 be dissolved by rain-water that has oozed through vegetable soil or 

 has soaked fallen leaves or other vegetable matter, and thereby be- 

 come saturated with carbonic acid. 



Where the petroleum finds a crevice leading to such cavities it 

 must creep through it and fill the space, thereby forming one of the 

 underground reservoirs supplying those pumping wells that have 



