OF VOLCANIC ROCKS. 83 



resulting events, exerted a powerful reaction upon the further promotion and accelera- 

 tion of those motions of the crust by which the irregularities of level of the surface were 

 produced. This may be inferred, in the first place, from the fact, that in and along 

 volcanic belts, elevation and plication of sediments have taken place, probably in every 

 instance ; both modes of disturbance have affected, partly sediments anterior in age 

 to the eruptive action, and partly such as were contemporaneous with and posterior to 

 it ; and there may be frequently noticed a diminution of the intensity of these dis- 

 turbances, in an inverse ratio with the distance from the volcanic belt. 29 But besides 

 this fact, observation has placed it beyond doubt, that in the second half of the Ter- 

 tiary period, and after it, a greater aggregate amount of relative, and probably too of 

 absolute elevation has taken place generally over the globe than had been the case for 

 ages before. The altitude of the rocks composing the summits of the highest moun- 

 tain ranges and table-lands of the present time, exceeds that which they had in the 



29 This evident connection, in volcanic regions, of elevation and plication, as cause and effect, appears to be one of 

 the gravest objections against the general validity of the theory of Mr. Hall on the mode of formation of mountain ranges, 

 though the incongruity of an explanation of the granitic wedges by metamorphism in situ, with the chemical nature of 

 granitic rocks, the total insufficiency of a gradual sedimentary deposition for accounting for a foot-for-foot subsidence of 

 large areas, and the difficulty of finding any natural cause of the assumed fact that the belts of greatest amplitude within 

 each area of subsidence should have been elevated to mountain ranges in preference to other neighboring parts, are objec- 

 tions of scarcely less weight. It is a well-established fact, that the Alps existed as a mountain range, at least from the time 

 of the Jurassic period (though probably long before it), and have gained in height in later periods. Yet the Jurassic and 

 Cretaceous strata are as much contorted as are those of more ancient age, and the same is true in a still greater measure 

 of the Tertiary Flysch. If, therefore, Mr. Vose, in his Orographic Geology (p. 68), arrives at the conclusion : " So far as 

 we have examined the facts, no other hypothesis than that of the slow sinking of vast masses of yielding sediments can at 

 all satisfactorily account for the plication and other evident effects of a compressive force so invariably exhibited in moun- 

 tain districts," the more recent formations of the Alps confirm this view as little as the above-mentioned example regarding 

 the volcanic belts. It may be mentioned, in this connection, that, while Mr. Hall's theory would necessarily imply an 

 increase in the intensity of plication towards the axis of greatest accumulation, which is considered by him to have been 

 coincident, after the elevation, with the axis of the mountain ranges, this conclusion is not confirmed by the structure of 

 the Alps. In Vorarlberg and northern Tyrol, which offer probably, of all parts of the Alps, the most normal and regular 

 structure, all strata, commencing with those of Triassic age, are least disturbed where they are nearest to the axis, and the 

 plication increases towards the foot of the range. Superposition, by contortion, of older formations on those of more recent 

 origin, is encountered with increasing frequency of instances and growing distinctness, in crossing the parallel waves from 

 those nearest to the axis, towards the northern foot of the range. Another objection, nearly related to the foregoing, is 

 this, that, granting that a trough-like depression would be able to cause plication by the settling of the yielding strata 

 towards the axis of greatest subsidence, the waves thereby formed would have their steeper inclination towards that axis, 

 and their flatter slopes directed outwardly, since a greater resistance would oppose the motion of the lower strata than 

 would be offered to the higher ones. The analogous instance, to which reference has been made, of a wave rolling towards 

 the beach, shows plainly the effect of the retardation suffered by the lower portion of the water by friction, in the steepening 

 of that side nearest to the place towards which motion is directed, and the final throwing over of the top in the same direc- 

 tion. A familiar instance, which repeats more exactly, though on a small scale, the conditions suggested by Mr. Hall's 

 theory, is afforded when a viscous mass, such as resin or pitch, moves downward on a slightly inclined plane. If the neces- 

 sary conditions are given for the formation of waves, their steeper sides will always be on the lower end, or, on that which 

 would be nearest to the axis of a trough-like depression. It is obvious that this shape is the reverse of that of the waves 

 of plication which Mr. Rogers and others have observed, and on which even Mr. Hall's theory is partially founded. This 

 theory, which was first proposed for a mountain range in which recent formations do not occur, and where, therefore, 

 not all of these objections could be raised, may, if somewhat modified, have its limited applicability for partially explaining 

 the mode of formation of certain mountain ranges ; but numerous arguments appear to preclude its general applicability, 

 especially so in the case of the European Alps, and probably not less in the case of the Himalaya. It appears, too, to be 

 quite inadmissible in the case of the Rocky Mountains ; and as regards the Sierra Nevada, only a chain of the most arbi- 

 trary assumptions would be able to give it an apparent validity for the explanation of any features in the structure of that 

 range. 



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