xxxii Appendix. 
upper line of which represents the thickness of the formation in feet, and the 
lower the proportionate compression :— 
| 
Thickness se ae 5,000 feet 10,000 feet 20,000. feet 25,000 feet 
Compression .. > i ian 35 w 
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 
ees ET compat of folded and contorted strata, 
