ON GEOLOGY. 



07 



iiaps, be held the reviver of the Plutonic system, which has «ince, 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, Professor 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 con- 

 siderably in favour of the latter class of philosophers, 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 reproduc- 

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

 renovation. Of decay induced by the action of hght, air, and other gases, 

 rain and other waters, upon tlie hardest rocks, by which they are worn 

 down, and their particles progressively carried towards the ocean, and ulti- 

 mately deposited in its bed ; and renovation^ by means of an immense 

 subterranean heat, constantly 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 afterwards 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 thi^ 

 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 fire 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 assist- 

 ance of fuel or of vital air. Friction is a source of heat, unhmited for 

 what we know in its extent ; and so, perhaps, are other operations chemical 

 and mechanical ; nor are either combustible substances or vital air con- 

 cerned 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 independent 

 of the substance just mentioned ; and though the heat would not calcine a 

 metal, nor even burn a piece of v/ood, without oxygenous gas, it would 

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

 ofthatgas."! 



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

 derable accession of power from the vast superincumbent weight that is 

 perpetually 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 carbonic acid of carbonate of lime, for example, are rendered incapable 

 of flying off, a much less quantity of actual heat is sufficient for the purpose 

 of fusion than when such gases, freed from a heavy compression, can es- 

 cape with facility. Now, the subterranean heat being supposed to exist 

 at prodigious depths below the surface, the substances on which it operates 



♦ Illttstrations of the Huttonian Theory of the Earth, Edinb, 18G2. 

 t Illiistratioiis of the Huttontan Theory, &c. 



