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results, whether they be chemical, or mechanical, or a compound of 

 both. But let any new and unknown condition be introduced, and 

 the results are not only changed, but are often the very contrary of 

 what we should have at first anticipated. Let it again be con- 

 sidered within what narrow limits we have the power of modify- 

 ing the conditions of any physical experiment, and how little we 

 still know of those mysterious imponderable agents which co-exist 

 perhaps, with gravitation, and unquestionably play their part in 

 every change and every combination — and we must see the utter 

 hopelessness of bringing under the definite calculations of any me- 

 chanical law, those mighty combinations still going on in the great 

 laboratory of nature. 



Of the origin of volcanic forces we know nothing : but we do 

 know that they are the irregular secondary results of great masses 

 of matter obeying the primary laws of atomic action — that they 

 differ in their intensity — are interrupted in their periods — and are 

 aggravated or constrained by an endless number of causes, external 

 and purely mechanical. Of all modes of material combination, 

 those of which I now speak are perhaps the most complicated. 

 To assume, then, that volcanic forces have not only been called 

 into action at all times in the natural history of the earth, but also, 

 that in each period they have acted with equal intensity, seems to 

 me a merely gratuitous hypothesis, unfounded on any of the great 

 analogies of nature, and I believe also unsupported by the direct 

 evidence of fact. This theory confounds the immutable and pri- 

 mary laws of matter with the mutable results arising from their 

 irregular combination. It assumes, that in the laboratory of nature, 

 no elements have ever been brought together which we ourselves 

 have not seen combined ; that no forces have been developed by their 

 combination, of which we have not witnessed the effects. And what is 

 this but to limit the riches of the kingdoms of nature by the poverty 

 of our own knowledge ; and to surrender ourselves to a mischie- 

 vous, but,not uncommon philosophical scepticism, which makes us 

 deny the reality of what we have not seen, and doubt the truth of 

 what we do not perfectly comprehend ? 



Into the solution of the great problem of the heavenly bodies, 

 there enter only a few simple and unchangeable mechanical ele- 

 ments, and the conclusions are of a simplicity corresponding to the 

 simplicity of the premises. All the celestial movements return into 

 themselves ; and even the most complex of the deviations pro- 

 duced by mutual perturbation, are confined within narrow limits, 

 and are completed in secular periods. The solution of this problem 

 is incontestably the greatest triumph of exact science. But with 

 what semblance of physical truth can we apply such mathematical 

 results as these to the great phenomena of geology — where the com- 

 binations are mutable and indefinite — where we have no vestige of 

 returning periods — and where the fixed elements of force are either 

 unknown or imperfectly comprehended ? 



If all the complex groups of crystalline and stratified rocks; if, 

 in a word, all the material things existing on the surface of the 



