during the Cooling of Intrusive Masses of Granite. 237 



treated of by Mr. W. Barlow in a paper read before the 

 Geological Society, which, although the author may claim 

 rather too much for his favourite force, shows in many ways 

 how, by simple gravitation, elevated masses of the earth 

 produce lateral pressure*. 



We thus see that while expansion-forces below the surface 

 of the earth produce both lateral compression, uplift, or 

 lateral creep, the effect of gravitation is to intensify the 

 lateral pressure and movement near the surface and to keep 

 the materials closely in contact during and after contraction. 

 Some of the critics of my theory of the origin of mountain- 

 ranges have found a difficulty in understanding how, if the 

 rise of the isogeotherms produces uplift by expansion, the fall 

 of the isogeotherms does not produce an equivalent sinking 

 by contraction and obliterate the range. 



The question is really fully answered in the book itself, in 

 which it is shown that the materials of a mountain-range 

 are by the eflfects of expansion, or a series of expansions, 

 moved towards and piled up into the range, and that they 

 cannot be drawn back and spread out in the areas from 

 which they have moved. Nothing, in fact, can remove them 

 save denudation. 



The effect of contraction is to produce normal faulting, 

 and this occurs most largely in the areas from which the 

 materials of the range have travelled. The mode of action 

 is really in one aspect very fairly shown in the experiments 

 on the expansion of metal plates, of which I have given many 

 illustrations. The total decrease of bulk of the section of the 

 earth's crusf, out of which the mountain- range has been 

 elaborated, may eventually equal the previous increase which 

 has produced the range, but the form is entirely altered. 

 Subsidence may bring the sea back to the foot of the mountains, 

 or, carried further, may make islands of them ; but it cannot 

 obliterate them any more than the subsidence of the ground 

 on which a volcano stands can obliterate it. 



A mountain-range grows upwards by compression, a 

 volcano principally by accretion ; but they both indicate, 

 though in different ways, a redisposition of materials which 

 it takes denuding agencies geological ages to efface. 



* Q. J. G. S. November 1888, pp. 783-796. 



