114 R. M. Deeley — Mountain Building. 



range ; for as long as the rocks below are of low density the mountain 

 mass will rise as fast as material is removed by denuding influences. 



The cause of the variations in the density of the deep-seated 

 portions of the earth's crust is as yet uncertain ; but we are entitled 

 to regard the crust as floating upon a liquid stratum of great viscosity, 

 or a very plastic solid stratum. 



It may not be out of place to explain what is here meant by 

 plasticity and viscosity. 



Although it is certain that our present knowledge of the properties 

 of fluids and solids, from the physical point of view, is by no means 

 complete, it will not be out of place to consider some of the very 

 complex phenomena they exhibit, and which have a bearing upon 

 geological problems. The subject is, indeed, one of, very great 

 importance to the engineer and physicist, for to the former the 

 physical properties of solids have to be considered as far as they affect 

 the stability of all kinds of structures, whilst in the case of the 

 latter they have to be borne in mind when dealing with the question 

 of the stability of mountain ranges, etc. 



One very frequently hears semifluids spoken of, and there are 

 many who are of opinion that there is a regular transition of the 

 solid into the liquid state, or that .there are a large number of 

 substances which can be arranged in sucli an order that they show 

 a transition from the solid to the liquid state. The idea that 

 liquidity is only a matter of degree was well expressed by Tyndall,^ 

 who writes : " What was the physical condition of the rock when 

 it was thus bent and folded like a pliant mass ? Was it necessarily 

 softer than it is at present ? I do not think so. The shock which 

 would crush a railway carriage, if communicated at once, is harmless 

 when distributed over the interval necessary for the pushing in of the 

 buffer. By suddenly stopping a cock from which water flows you 

 may burst the conveyance pipe, while a slow turning on of the cock 

 keeps all safe." All this is more plausible than sound. He then 

 goes on : " Might not a solid rock by ages of pressure be folded as 

 above ? It is a physical axiom that no body is perfectly hard, 'none 

 perfectly soft, none perfectly elastic. The hardest body subjected to 

 pressure yields, however little, and the same body when the pressure 

 is removed cannot return to its original form. If it did not yield in 

 the slightest degree it would be perfectly hard; if it could completely 

 return to its original shape it would be perfectly elastic." 



" Let a pound weight be placed upon a cube of granite; the cube 

 is flattened, though in an infinitesimal degree. Let the weight be 

 removed, the cube remains a little flattened ; it cannot quite return 

 to its primitive condition. Let us call the cube thus flattened No. 1. 

 Starting witb No. 1 as a new mass, let the pound weight be laid 

 upon it ; the mass yields, and on removing the weight it cannot 

 return to the dimensions of No. 1 ; we have a more flattened mass, 

 No. 2. Proceeding in this manner, it is manifest that by a repetition 

 of the process we should produce a series of masses, each succeeding 

 one more flattened than the former. This appears to be a necessary 

 consequence of the physical axiom referred to above. 

 ^ Glaciers of the Alps, 1860, p. 9. 



