THE NONMETALLIC MINERALS. 435 



rock of porous nature to act as a reservoir, the petroleum-bearing 

 rocks are mostly sandstones, though not uniformly so. Petroleums 

 are found in California and Texas in Tertiary sands; in Colorado in the 

 Cretaceous; in West Virginia both above and below the Crinnoidal 

 (Carboniferous) limestones; in Pennsylvania in the " mountain " sands 

 (Lower Carboniferous) and the Venango sands (Devonian); in Canada 

 in the Corniferous (Lower Devonian) limestones; in Kentucky in the 

 Hudson River shales (Lower Silurian), and in Ohio in the Trenton 

 limestone. (See series illustrating geological distribution.) 



In some instances petroleum oozes naturally from the ground, form- 

 ing at times a thin layer on the surface of pools of water, whence in 

 times past it has been gathered and used for chemical and medicinal 

 purposes. The so-called "Seneca oil" thus used some fifty or sixty 

 years ago was thus obtained from a spring in Cuba, Allegany County, 

 in New York. The immense supply now demanded for commercial 

 purposes is, however, obtained altogether from artificial wells of vary- 

 ing depths, and which are in some cases self-flowing, while in others 

 the oil is raised by means of pumps. Wells of from 500 to 1,500 feet 

 in de.pth are of common occurrence, while those upwards of 2,000 feet 

 are not rare. The principal sources of petroleum are in the United 

 States New York, Pennsylvania, and Ohio, with smaller fields in 

 West Virginia. Kentucky, Tennessee, Indiana, Texas, Colorado, and 

 California. The chief foreign source is the Baku region on the Cas- 

 pian Sea, and Galicia, in Austria. 



Uses of petroleum. The earty uses of petroleum in America seem to 

 have been for medicinal purposes only (Specimen No. 59834, U.S.N.M. , 

 from Kentucky). The oil as pumped from the wells has but a limited 

 application in its crude condition excepting as a fuel, and owes its 

 great value to the large and varied series of derivatives which it 

 yields. A discussion of the methods employed in obtaining these 

 derivatives belongs properly to the department of chemical technology 

 and can not be dwelt upon here. It must suffice for present pur- 

 poses to say that the treatment as ordinarily carried out at present 

 involves a process of destructive distillation whereby the crude mate- 

 rial, heated under pressure, is resolved into a variety of products of 

 different densities, and varying from gaseous through liquid to solid 

 forms. Prominent among these derivatives may be mentioned, aside 

 from the gaseous compounds, rhigolene, gasoline, naphtha, benzine, 

 kerosene, various lubricating oils, paraffin, and the solid residues (coke, 

 etc.). Various pharmaceutical compounds are prepared from petro- 

 leum products, many of which are well known to the public, as vase- 

 line, cosmoline, etc. It is also used as a basis for ointments and in 

 soaps. 



The accompanying map (Plate 25) from the reports of the Tenth 

 Census will serve to show the distribution of petroleum and allied 



