Notices of Memoirs — Hutton, on Mountains. 173 



A first inspection of this table will give tlie impression that these 

 compressions are not nearly enough to account for the contortions 

 we see in mountain districts, but I believe that our ideas of contor- 

 tions are very incorrect, owing to the necessarily exaggerated sections 

 that accompany geological descriptions. The only sufficiently ac- 

 curate section that I have been able to see is Professor Ramsay's 

 beautiful section through Snowdon, in North Wales, and after care- 

 fully measuring it, and allowing for the faults and intrusive rocks, I 

 find that the compression in this mountainous district is one-sixteenth. 

 We must also remember that the contortions that we now see are 

 the sum of all the compressions that have taken place at various 

 times, for the rocks after being bent do not straighten out again on 

 being stretched, but elongate by faulting. A considerable amount 

 of the contortions of the lower beds of a formation will also be a 

 necessary consequence of elevation by expansion, for during eleva- 

 tion the lower beds will not be able to expand so much as the upper 

 ones of the arch, although much more heated. 



The subsidence of an area caused by the weight of newly-deposited 

 matter will compress the underlying superheated rocks, and, as 

 explained at the commencement of the lecture, this will caxise an 

 increase of upward pressure in the surrounding areas. This increase 

 of upward pressure will caiise elevation in the surrounding districts, 

 the rocks will be subjected to tension, and fissures will be formed. 

 Up these fissures the superheated rocks of the interior will rise, and 

 if they reach the surface will form volcanoes and overflow as lava 

 streams. In this way mountains of quite a different character to 

 those we have lately considered will be formed. 



I have now explained to you the theory of Messrs. Herschel and 

 Babbage in its simplest form, but in nature we should rarely find 

 this simplicity. These two great powers — expansion by heat, and 

 increase of weight — would sometimes combine and sometimes inter- 

 fere with each other. Complications would also arise from the 

 different degrees of fusibility, conductivity, porosity, and expansion 

 of rocks, while the changes in physical geography caused by the 

 changes in the position of the land would constantl}^ alter the mean 

 temperature of the surface, so that very complex phenomena might 

 result from these simple causes. 



To sum up. Mountain chains are of two kinds. The first, of 

 which tlie Alps may be taken as a type, are composed of folded and 

 contorted strata, generally associated with metamorphic and granitic 

 rocks. These have been formed by heavy argillaceous deposits, 

 causing subsidence and contortion, which have been subsequently 

 elevated by the superposition of calcareous beds. The second kind, 

 of which the Andes may be taken as the type, are composed of nearly 

 horizontal strata, generally associated with volcanic rocks.^ These 

 have been formed by the up)M'ard pressure of the underlying rocks 

 caused by the subsidence of adjoining areas, and owe their height 



1 See also Darwin " On Volcanic Phonomeua in South America." — Trans. Geol. 

 Soc, 2nd series, p. 601. 



