ON AQUEOUS AND IGNEOUS METEORS. 5G3 



Convulsions of these kinds must have very materially influenced the 

 disposition of the strata of the earth, as well as the form of its surface ; 

 but it is by no means fully determined how far such causes have been 

 concerned, or how far the effects are to be attributed to the intermediation 

 of water only. Mineralogists and geologists have been principally divided 

 into two classes with respect to their theories of the earth, some maintain- 

 ing the Vulcanian, and some the Neptunian hypothesis. It appears to be 

 impossible to decide with any certainty between these opposite opinions ; 

 nor is it perhaps of much consequence for any purpose of practice, or even 

 of science. The Neptunians are certainly able to establish their own 

 theory positively, and to prove that the fluid parts of the earth and sea 

 must have been very materially concerned in producing the changes which 

 have happened to the solid parts ; but it may be difficult for them to con- 

 fute the assertion, that heat, whether caused by volcanos or otherwise, has 

 also been a very powerful agent in these operations, and in some cases the 

 joint effects of heat and of increased pressure appear to have been con- 

 cerned in giving to minerals of different kinds their actual form ; although 

 on the whole it seems probable that the operation of heat has been much 

 more limited than that of aqueous solutions and precipitations. Mr. 

 Davy has also very justly inferred, from his experiments with the battery 

 of Volta, that the effects of the electricity excited by means of chemical 

 changes within the earth, have probably been very materially concerned in 

 the gradual formation of a variety of mineral productions. 



The arguments for establishing the general fact, that great convulsions 

 have actually happened to the earth, are too well known to require minute 

 examination : the variety of fossil substances, many of them marine pro- 

 ductions, and some almost preserving a recent appearance, that are found 

 in mountains remote from the sea, are undeniable proofs that the levels of 

 the earth's surface must have undergone considerable changes ; although 

 some philosophers are of opinion, that such of the primary mountains as 

 are above 6 or 700 feet high, have never been wholly covered by the sea. 

 It is not at all easy to explain the change of climate, which some of these 

 circumstances appear to indicate ; the remains of animals inhabiting hot 

 countries, and the marine productions of hot climates, which are frequently 

 found in high northern latitudes, would induce us to suspect, that the posi- 

 tion of the earth's axis was at a former time very different from its pre- 

 sent position : and we can scarcely assign any other probable cause for 

 this change, than the casual interference, and perhaps incorporation, of a 

 comet with the earth. The probabilities of such an event, in the whole 

 course of time, are however so small, that we have no reason to be appre- 

 hensive of the chance of its occurring in future, for it is not enough that a 

 comet should approach so near to the earth as to be very powerfully at- 

 tracted by it, its motion must also be directed almost in a straight line 

 towards the earth ; otherwise it might only be inflected into a new orbit, 

 and go off again, without having caused any other disturbance than a 

 partial overflow of the sea. 



The face of the globe has also been very materially changed in the 

 course of ages, by the gradual operation of the sea and of rivers. The sea 



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