406 SUMMARY OF WORK. 



that was not damped by the torrents of rain that fell 

 as the travellers threaded their way down one of the 

 ravines. 



Even had this journey never been made, Prestwich's 

 sound views and wide sympathies in every department 

 of his favourite science would not probably have 

 allowed him to leave the field of volcanic geology un- 

 trodden. He had evidently reflected much on the 

 subject before he contributed, in 1885, three short but 

 suggestive papers to the Royal Society. In the first 

 of these he discussed the various recorded observations 

 of underground temperature, and concluded that the 

 rise of the thermometer amounts to an average of 1 

 Fahr. for every 48 feet of descent. He further sug- 

 gested that the abnormally high temperatures found 

 in piercing the Alps for the construction of railway 

 tunnels might be the residue of the heat caused by the 

 intense lateral pressure and crushing of the rocks 

 which accompanied the last elevation of the mountain- 

 chain [112]. Pursuing this idea, he was led to speculate 

 on the probable cause of the metamorphism observable 

 among mountain-ranges in strata which, upon the sur- 

 rounding plains, have undergone no alteration [114]. 

 He connected the change with the great development 

 of heat during the process of mountain-making. Rea- 

 soning from the results of Mallet's experiments on 

 rock-crushing, he contended that the effects of this 

 increased temperature would vary with compressibility, 

 some rocks being made three times hotter than others 

 under the same strain. In this way he accounted for 

 the local character of the metamorphism, and for its 

 much more marked development in some strata than in 

 others. In the third memoir [113], he controverted the 

 common assumption that the expulsion of lava at a 



