The theories of its origin suggested by Reichenbach, Berthelot, Mendeleeff, Peckham, and others, made no attempt to account for the exceeding variety in its chemical composition, in its specific gravity, its boiling points, etc., and are all founded on some hypothetical process which differs from any with which we are acquainted; but modern geologists are agreed that, as a rule, the records of the earth’s history should be read in accordance with those laws of nature which continue in force at the present day, e.g., the decomposition of fish and cetaceous animals could not now produce oil containing paraffin.  Hence we can hardly believe it was possible thousands or millions of years ago, if it can be proved that any of the processes of nature with which we are familiar is calculated to produce it.

The chief characteristics of petroleum strata are enumerated as: 

    I. The existence of adjoining beds of limestone, gypsum, etc.

    II.  The evidence of volcanic action in close proximity to
    them.

    III.  The presence of salt water in the wells.

I. All writers have noticed the presence of limestone close to petroleum fields in the United States and Canada, in the Caucasus, in Burma, etc., but they have been most impressed by its being “fossiliferous,” or shell limestone, and have drawn the erroneous inference that the animal matter once contained in those shells originated petroleum; but no fish oil ever contained paraffin.  On the other hand, the fossil shells are carbonate of lime, and, as such, capable of producing petroleum under conditions such as many limestone beds have been subjected to in all ages of the earth’s history.  All limestone rocks were formed under water, and are mainly composed of calcareous shells, corals, encrinites, and foraminfera—­the latter similar to the foraminfera of “Atlantic ooze” and of English chalk beds.  Everywhere, under the microscope, the original connection of limestone with organic matter—­its organic parentage, so to speak, and cousinship with the animal and vegetable kingdoms—­is conspicuous.  When pure it contains 12 per cent. of carbon.

Now petroleum consists largely of carbon, its average composition being 85 per cent. of carbon and 15 per cent. of hydrogen, and in the limestone rocks of the United Kingdom alone there is a far larger accumulation of carbon than in all the coal measures the world contains.  A range of limestone rock 100 miles in length by 10 miles in width, and 1,000 yards in depth, would contain 743,000 million tons of carbon, or sufficient to provide carbon for 875,000 million tons of petroleum.  Deposits of oil-bearing shale have also limestone close at hand; e.g., coral rag underlies Kimmeridge clay, as it also underlies the famous black shale in Kentucky, which is extraordinarily rich in oil.