ORIGIN AND DURATION OF PETROLEUM. 277 



Limestone, although not porous like shales and sand- 

 stones, is specially well adapted for storing large subter- 

 ranean accumulations, on account of the grent 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 especially 

 abundant in the " carboniferous limestone" which underlies 

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

 stone may be dissolved by rain-water that has oozed through 

 vegetable soil or has soaked fallen leaves or other vegetable 

 matter, and thereby become 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 yielded such abundance for a while 

 and then become dry. But if this theory is correct it does 

 not follow that the drying of such a well proves a final 

 stoppage of the supply, for if the cavity and crevice are left, 

 more oil may ooze into the crevice and flow into the cavity, 

 and this may continue again and again throughout the 

 whole oil district so long as the surrounding feeders of per- 

 meable strata continue saturated, or nearly so. The mag- 

 nitude of these feeding grounds may far exceed that of the 

 district wherein the springs occur, or where profitable wells 

 may be sunk, seeing that the localizing of profitable supply 

 depends mainly on the stoppage of further oozing away by 

 the action of the impermeable barrier. 



A well sunk into the oozing strata itself would receive a 

 very small quantity, only that which, in the course of its 

 passage came upon the well sides, while at the junction be- 

 tween the permeable and the impermeable rocks the accu- 

 mulation may include all that reached the whole surface of 

 such junction or contact many square miles. 



To test this theory thoroughly it would be necessary to 

 make borings, not merely at the wells, but in their neigh- 

 borhood, where the porous rocks dip towards the limestone, 

 and to bring up sample cores of these porous rocks, and 

 carefully examine them. Dr. Sterry Hunt has done this in 

 the oil-yielding limestone rocks of Chicago, but not in those 

 of the nearest coal-measures. 



