49 



OBSERVATIONS ON THE CAUSES OF ELEVATION 



AND SUBSIDENCE OF THE EARTH'S CRUST. 



By R. M. Johnston, F.L.S. 



Any enquiry into the causes which have produced, and are 

 still producing, mountains and mountain chains cannot be 

 satisfactorily carried out apart from the study of the causes 

 which originally produced the grand irregularities of surface, 

 which primarily determined the original areas of continents 

 and oceans. 



In offering any opinion of my own upon the merits or 

 defects of the rival hypotheses assumed by so many eminent 

 physicists to account for elevation and subsidence, I hope it 

 will be conceded that I do so with much diffidence ; for where 

 so many eminent men fail to be satisfied with each other's 

 views on a subject which necessarily rest so largely on 

 arbitrary values for unknown data, it is obvious that any 

 view which I may tend to favour must in my own mind be 

 attended with a large measure of doubt, and on some obscure 

 points my judgment may be suspended entirely. As a novel 

 theory, however, has recently been referred to by Mr. F. 

 Danvers-Power, relating to the elevation of mountain chains, it 

 may be of some interest at this time to enter more fully into the 

 consideration of those hypotheses of causation which have 

 gained the most favour among physicists and geologists. 

 Of course the correct observation of effects and the true 

 interpretation of immediate or primary causes are very 

 different things. The geologist, as such, is entitled to speak 

 with authority as regards the former, but undoubtedly the 

 correct interpretation of causation is more purely within that 

 realm of seience where the phj sicist, as such, has the greater 

 claim to be heard. The geologist may best know what are 

 the magnitude and characteristics of certain obvious changes 

 in a given region, but his interpretation of the cause or 

 causes at work — not so open to observation — may be liable 

 to errors from which the knowledge of the physicist might 

 have saved him. The apparent cause to the geologist may, 

 to the mind of the skilled physicist, be altogether objection- 

 able when tested by his more exact methods for determining 

 its nature and efficiency. With some the mere parallelism 

 of certain phenomenal features — great ocean boundaries and 

 certain mountain chains — often appear to stand as cause and 

 effect. This may be so ; but is it true ? The answer to this 

 query will be given hereafter. As it is desirable before 



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