Correspondence — Mr. T. Mellard Reade. 569 



I hope, therefore, that Prof. Prestwich will accept my assurance 

 that I had no intention of suggesting that he was in any way 

 indebted to Phillips. I regret also that my reference to the Weald 

 is not sufficiently clear ; I said : " It is open to question whether the 

 Weald ever had the character of a mountain-region." The question 

 which I was considering was whether stones are ever worn by the 

 action of torrents so as to produce a groove in one side, and I placed 

 the remarks quoted above in a footnote. Of course the Weald must 

 at some time have been a region of comparative elevation; but con- 

 sidering the nature of the materials of which it was composed, and 

 that denudation must have gone on during the period of gradual 

 elevation, it appeared to me permissible to doubt whether it was 

 ever a mountain-region in the sense in which, for instance, the 

 district of Snowdon is a mountain-region ; that is to say, a region 

 giving birth to numerous torrents. 



But in any case the argument in my paper would not be affected ; 

 and I have certainly no wish to raise a controversy on a matter in 

 regard to which Prof. Prestwich knows far more than I do. 



Thorndale, Ckaven Road, Eeading. 0. A. ShrUBSOLE. 



MOUNTAIN-MAKING BY TENSION. 



Sir, — Mr. Vaughan " having stated a new theory to account for 

 the inequalities of the Earth's Surface," and this theory being 

 dependant upon the tensile strength of the Earth's crust, I suggested 

 that he should favour us with some proof that the outer shell of the 

 Earth is sufficiently strong to do the work demanded of it. He 

 observed in his first paper,^ "It obviously follows that the outer 

 shell exerts a squeezing force upon the interior, and by compressing 

 the mass into a smaller volume increases its density." In my 

 communication to this Magazine ^ I pointed out that no tensile 

 stress that the Earth's crust could stand would be sufficient to 

 compress the materials of the interior of the Earth, stating in effect 

 that if the outer shell is assumed to be 30 miles thick, and of the 

 tensile strength of steel, it could not exert a pressure of half a ton 

 per square inch upon the interior without fracturing. 



Mr. Vaughan now says that he does not rely upon decrease of 

 volume due to pressure, but " upon the transference of material from 

 beneath a surface of great pressure to below a surface upon which 

 the pressure is much less." This is not very different to what I 

 understood of his theory from his first paper, and my calculation 

 was given merely to show what an exaggerated view Mr. Vaughan 

 held of the compressive powers of a contracting crust. Mr. Vaughan's 

 theory, so far as I can understand it, appears to be this : — Mountain 

 Ranges are produced by the differential tensile stresses of a shrinking 

 crust causing a local flattening of the Earth's curvature, and thus 

 compelling a flow of material from where the crust is strong enough 

 to prevent, to where it is weak enough to permit of, bulging. Now, 

 on the assumption that a shell of steel 30 miles thick represents the 

 tensile strength of the contracting crust — an exceedingly liberal 



1 Geol. Mag. 1894, p. 264. 2 j^i^^ p, 414, 



