MALLET'S THEORY OF VOLCANOES. 153 



tenable, suggested that there may be isolated subterranean 

 lakes of fiery matter, and that these may be the true seat of 

 volcanic energy. But such lakes could not maintain their 

 heat for ages, if surrounded (as the theory requires) by 

 cooler solid matter, especially as the theory also requires 

 that water should have access to them. It will be observed 

 also that none of the theories just described affords any 

 direct account of those various features of the earth's sur- 

 face mountain ranges, table-lands, volcanic regions, and so 

 on which are undoubtedly due to the action of subter- 

 ranean forces. The theory advanced by Mr. Mallet is open 

 to none of these objections. It seems, indeed, competent 

 to explain all the facts which have hitherto appeared most 

 perplexing. 



It is recognized by physicists that our earth is gradually 

 parting with ks heat As it cools it contracts. Now if this 

 process of contraction took place uniformly, no subterranean 

 action would result. But if the interior contracts more 

 quickly than the crust, the latter must in some way or other 

 force its way down to the retreating nucleus. Mr. Mallet 

 shows that the hotter internal portion must contract faster 

 than the relatively cool crust ; and then he shows that the 

 shrinkage of the crust is competent to occasion all the 

 known phenomena of volcanic action. In the distant ages 

 when the earth was still fashioning, the shrinkage produced 

 the irregularities of level which we recognize in the elevation 

 of the land and the depression of the ocean-bed. Then 

 came the period when as the crust shrank it formed corru- 

 gations, in other words, when the foldings and elevations 

 of the somewhat thickened crust gave rise to the mountain- 

 ranges of the earth. Lastly, as the globe gradually lost its 

 extremely high temperature, the continuance of the same 

 process of shrinkage led no longer to the formation of ridges 

 and table-lands, but to local crushing-down and dislocation. 

 This process is still going on, and Mr. Mallet not only 

 recognizes here the origin of earthquakes, and of the 

 changes of level now in progress, but the true cause of 



