Internal Cavities of the Earth. 221 



mediately after the formation of the carljoniferous series ? The 

 reduction of temperature at that epoch was inconsiderable ; tro- 

 pical 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 dis- 

 turbances which appear to have taken place about that period 

 in all other parts of the world, I aiti afraid our satisfaction would 

 be greatly diminished on finding that Gosau and Maestricht * 

 escaped unhurt. 



Be the cause what it may, the effect is certain. The tempe- 

 rature of the crust of the earth must have been higher when the 

 coal-measures were deposited than now, and we have reason to 

 think it was still higher at antecedent periods. That a con- 

 siderable 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 re- 

 garded as visionary ; and in the extent to which it was carried 

 by some of the old Cosmogonists it was so ; but thiat compara- 

 tively 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 conse- 

 quence 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 open- 

 ings, the vast extent of territory shaken simultaneously by their 

 convulsions, are so many proofs of communication below the 

 surface. The bulk of the ejected matter cannot.be less than 

 that of the concreted ejections which we see ; for at the tempe- 

 rature of fusion it is greater than at a lower temperature, and 

 for every foot of matter ejected it is necessary to provide a sub- 

 stitute 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 



• See the descriptions of these in GeoL Trans. 



VOL. XVII. NO. XXXIV. 0CT0BElM834. Q 



