74 



ON GEOLOGY. 



date, and both of them have been agitated in ancient as well as in modern 

 times with a considerable degree of warmth as well as of plausible argument. 



Among the ancients, Heraclitus seems to have headed the advocates for the 

 former theory, and Thales, or rather Epicurus, the supporters of the latter. 

 In what may be regarded as modern times, Hooke may, perhaps, be held the 

 reviver of the Plutonic system, which has since, as I have already observed, 

 been supported by the cosmological doctrines of Buffon and Dr. Herschel. 

 Its principal champions, however, in the present day are Dr. Hutton, Pro- 

 fessor Playfair,* and Sir James Hall ; names, unquestionably, of high literary 

 rank, and entitled to the utmost deference, but most powerfully opposed by 

 the distinguished authorities of Werner, whose system I have just glanced 

 at, Saussure, Kirwan, Cuvier, and Jameson, not to mention that the general 

 voice of geologists is very considerably in favour of the latter class of philo- 

 sophers, and consequently of the Neptunian or aqueous hypothesis. Let us, 

 then, take a brief view of each of these theories in their order. 



According to the former, or the Plutonic conjecture, heat is the great source, 

 not only of the original production, but of the perpetual reproduction of 

 things. This theory supposes a regular alternation of decay and renovation. 

 Of decay induced by the action of light, air, and other gases, rain, and other 

 waters, upon the hardest rocks, by which they are worn down and their par 

 tides progressively carried towards the ocean, and ultimately deposited in 

 its bed; and of renovation^ by means of an immense subterranean heat, con- 

 stantly present at different depths of the mineral regions; which operates in 

 the fusion and recombination of the materials thus carried down and contained 

 there, and afterward in their sublimation and re-exposure to view in new 

 strata of a more compact and perfect character. Hence, the existing strata 

 of every period consist, upon this theory, of the wreck of a former world, 

 more or less completely fused and elevated by the agency of violent heat, and 

 reconsolidated by subsequent cooling : of the general nature of which heat, 

 however, we are still left in a considerable degree of ignorance. "It is not 

 lire, in the usual sense of the word," observes Mr. Playfair, " but heat, which 

 is required for this purpose ; and there is nothing chimerical in supposing 

 that nature has the means of producing heat, even in a very great degree, 

 without the assistance of fuel or of vital air. Friction is a source of heat 

 unlimited, for what we know, in its extent ; and so, perhaps, are other ope- 

 rations, chemical and mechanical ; nor are either combastible substances or 

 vital air concerned in the heat thus produced. So, also, the heat of the sun's 

 rays in the form of a burning-glass, the most intense that is known, is inde- 

 pendent of the substance just mentioned ; and though the heat would not cal- 

 cine a metal, nor even burn a piece of wood, without oxygenous gas, it would 

 doubtless produce as high a temperature in the absence as in the presence of 

 that gns."t 



This subterranean heat, moreover, is supposed to derive a very considera- 

 ble accession of power from the vast superincumbent weight that is perpe- 

 tually pressing upon its materials ; in confirmation of which a variety of 

 curious experiments are appealed to, and especially a very ingenious set lately 

 carried into effect and described by Sir James Hall, by which it has been 

 rendered probable, that when the gases of any fusible substance, as the car- 

 bonic acid of carbonate of lime, for example, are rendered incapable of flying 

 off, a much less quantity of actual heat is sufiicient for the purpose of fusion 

 than when such gases, freed from a heavy compression, can escape with 

 facility. Now, the subterranean heat being supposed to exist at prodigious 

 depths below the surface, the substances on which it operates must be so 

 enormously compressed, as not only to render them easily fused, but in 

 many instances to prevent their volatilization after the fusion has taken 

 place ; and from this circumstance it is possible, we are told, to explain a 

 variety of appearances and qualities in minerals, and to answer a variety of 

 objections which would otherwise weigh heavy against the general theory. 



* Illustrations of the Huttonian Theory of the Earth. Edinb. 1802. t Ibi<J« 



