66 



phenomena of Melville Island, it might render inexplicable those of 

 the rest of the world. If we choose to change the axis upon which the 

 earth revolves, let us at least fix upon the best time fordoing it; now 

 what is that time? immediately after the formation of the carboniferous 

 series ? The reduction of temperature at that epoch was inconsider- 

 able; tropical plants and animals are found in the lias, in the oolite 

 series, in the chalk. A much more convenient time would be on 

 the first appearance of the tertiary rocks; but however satisfactory 

 it might be to trace to such a cause the violent changes and disturb- 

 ances which appear to have taken place about that period in all other 

 parts of the world, I am afraid our satisfaction would be greatly di- 

 minished on finding that Gosau and Maestricht* escaped unhurt. 



Be the cause what it may, the eflect is certain. The Temperature 

 of the Crust of the Earth must have been higher when the Coal- mea- 

 sures were deposited than now, and we have reason to think it was 

 still higher at antecedent periods. That a considerable degree of Heat 

 still exists, either partially or generally, at no great distance from the 

 surface, appears from thermal springs and volcanoes. 



I am aware that the doctrine of Internal Cavities has been regarded 

 as visionary ; and in the extent to which it was carried by some of the 

 old Cosmogenists it was so ; but that comparatively near to the surface, 

 there are, I do not say Vacuities, but large Spaces unoccupied by solid 

 matter, is not only probable, but almost proved. It seems, indeed, to 

 be a necessary consequence of the structure of the crust of the earth. 

 No miner has ever got to the bottom of a vein, and a vein itself is 

 often a half empty pipe or fissure. The correspondence of the 

 phases of distant volcanoes, the continuous ranges of their eruptive 

 openings, the vast extent of territory shaken simultaneously by their 

 convulsions, are so many proofs of communication below the sur- 

 face. The bulk of the ejected matter cannot be less than that of the 

 concreted ejections which we see^ for at the temperature of fusion 

 it is greater than at a lower temperature, and for every foot of matter 

 ejected, it is necessary to provide a substitute in the place which 

 it occupied. 



The continuous streams of lava which issued in Iceland, on one 

 occasion, attained the length of forty or fifty miles. But the bulk of 

 volcanic matter presented to view, does not enable us to form a 

 correct estimate of the quantity of matter ejected; we must take 

 further into account the combustible substances which have vanished, 

 the gases which have escaped, the dust and ashes which, projected 

 into the air, have fallen many miles distant from the place of explo- 

 sion f. Then only can we entertain a just idea of the Cavities that must 

 have been created in the interior of the earth by the escape of a mass 

 of matter competent to produce an Etna or a Chimbora^o. Such Ca- 

 vities are ill suited to support such Mountains ; La Metherie therefore 

 supposes Cavities to be at a distance, and volcanic matter to flow 



* See the descriptions of these places in Geol. Trans. 



t In 1783, a submarine Volcano off the coast of Iceland ejected so much 

 pumice that the ocean was covered to a distance of 150 miles, and Shi[is 

 were considerably ii:;pcdcd in their course. 



