have been built upon the surface of matter at a high temperature, 

 which subsequently contracted by slowly cooling down ; — that when 

 this contraction had reached a certain point, a fresh accession of 

 heat from some neighbouring volcano, by raising the temperature 

 of the beds again, produced a renewed expansion, and which re- 

 stored the temple to its present level. The periods at which these 

 events happened are then compared with various historic records. 



The second part of this letter contains some views, respecting 

 the possible action of existing causes, in elevating continents and 

 mountain-ranges, which occurred to the author in reflecting on 

 the preceding explanation. He assumes as the basis of this rea- 

 soning the following established facts: 



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

 point, the temperature increases. 



2. That solid rocks expand by being heated; but that clay 

 and some other substances contract under the same circumstance. 



3. That different rocks and strata conduct heat differently. 



4. That the earth radiates heat differently, or at different parts of 

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

 with deserts, or with water. 



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

 stantly changing the condition of the surface of the globe. 



Mr. Babbage then proceeds to remark, that whenever a sea or 

 lake is filled up, by the continual wearing down of the adjacent 

 lands, new beds of matter, conducting heat much less quickly than 

 water carries it, are formed ; and that the radiation, also, from the 

 surface of the new land, will be different from that from the water. 

 Hence, any source of heat, whether partial or central, which pre- 

 viously existed below that sea, must heat the strata underneath 

 its bottom, because they are now protected by a bad conductor. 

 The consequence must be, that they will raise, by their expan- 

 sion, the newly formed beds above their former level; — and thus 

 the bottom of an ocean may become a continent. The whole ex- 

 pansion, however, resulting from the altered circumstances, may 

 not take place until loi^g aller the filling up of the sea; in which 

 case its conversion into dry land will result partly from the filling 

 up by detritus, and partly from the rise of the bottom. As the 

 heat now penetrates the newly formed strata, a different action 

 may take place ; the beds of clay or sand may become consoli- 

 dated, and may contract instead of expanding. In this case, 

 either large depressions will occur within the limits of the new 

 continent, or, after another interval, the new land may again sub- 

 side, and form a shallow sea. 'J'his sea may be again filled up 

 by a repetition of the same processes as before : — and thus alter- 

 nations of marine and freshwater deposits may occur, having inter- 

 posed between them the productions of dry land. 



Mr. Babbage's theory, therefore, may be thus briefly sfated. — 

 In consequence of the changes actually going on at the earth's 

 surface, the surfaces of equal temperature within its crust, must 



