206 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



state of activity, throwing up pumice occasionally from SOO to 600 

 feet high, I was desirous of ascertaining the depth of the great 

 crater. Accordingly, having taken the angles of certain points and 

 measured, on the rough lava plain at its bottom, a base of 330 feet, 

 I left a walking-stick fixed upright to mark its termination. After 

 taking the angles at the opposite end I remeasured the base, and 

 found it 331 feet ; but on returning the walking-stick was in flames. 

 In many of the crevices a foot or two in depth, the lava was red-hot. 

 The bottom of the small crater in which the pasty semi-fluid lava was 

 tossed about, was, I should think from memory, not more than fifty 

 feet below the spot on which I stood. The depth of the plain at 

 the bottom of the crater below the lowest part of its upper edge was, 

 on the 3rd June 1 828, about 505 feet. 



I had been surprised at the small distance (two or three feet only) 

 which separated the red-hot lava from the less heated surface on 

 which I had been walking, and had carried on my trigonometrical 

 operations, without experiencing much inconvenience from the heat. 

 The view of the melted lava had however disappointed my expecta- 

 tions : instead of possessing fluidity, it ought rather to be described as 

 a pasty viscous mass, having some degree of toughness, of an uneven 

 surface, occasionally pushed up by a force from below which caused 

 elevations that very slowly subsided, or more frequently were removed 

 or interfered with by succeeding efforts of the same force. 



85- The following explanation of the origin of the changes which 

 have continually taken place in the forms and the levels of large por- 

 tions of the earth's surface at many distant periods of time, and which 

 appear still to continue their slow but certain progress, arose from 

 the examination of the temple of Serapis, which has been detailed 

 in the former part of this paper. 



The theory rests upon the following principles : — 



1st. That as we descend below the surface of the earth at any point, 

 the temperature increases. 



2nd. That solid rocks expand by being heated, but that clay and 

 some other substances contract under the same circumstances. 



3rd. That different rocks and strata conduct heat diff'erently. 



^th. That the earth radiates heat differently from different parts 

 of its surface, according as it is covered with forests, with mountains, 

 with deserts, or with water. 



5th. That existing atmospheric agents and other causes are con- 

 stantly changing the condition of the earth's surface, and that, assisted 

 by the force of gravity, there is a continual transport of matter from 

 a higher to a lower level. 



The existence of the four latter causes has long been fully ad- 

 mitted : the only one on which any uncertainty rests is the first. All 

 measures which have been made of the increase of the earth's heat 

 as we descend below its surface concur in pointing out the fact, al- 

 though, as might be expected, almost every case gives a diff'erent 

 amount of descent for an elevation of temperature of one degree of 

 Fahrenheit. In tracing out some of the consequences which neces- 

 sarily result from the continued action of these five causes, it will be 



