xxxu 



Appendix. 



upper line of which represents tlie thickness of the formation in feet, and the 

 lower the proportionate compression : — 



Thickness 

 Compression 



5,000 feet 



1 



1000 



10,000 feet 



500 



20,000 feet 



1 



300 



25,000 feet 



1 



200 



A. first inspection of this table will give the 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 contortions are very- 

 incorrect, owing to the necessarily exaggerated sections that accompany 

 geological descriptions. The only sufficiently accurate section that I have 

 been able to see is Professor Ramsay's beautiful section through Snowdon, in 

 North Wales, and after carefully 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 elevation 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 cause an increase of upward pressure 

 in the surrounding areas. This increase of upward pressure will cause 

 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 over- 

 flow 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 interfere 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 constantly alter the mean tempera- 

 ture 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 the 

 Alps may be taken as a type, are composed of folded and contorted strata, 



