230 Revieivs — T. Mellard Reach — On Mountain Ranges. 



mountain-chain, is the rise of the isogeotherms, and consequent 

 increase of temperature " [and expansion] " of the new sedimentaries, 

 and that portion of the old crust that they overlie" (p. 326). It 

 will therefore be seen that the theory is founded on what has 

 been termed the Herschel-Babbage hypothesis, that, where thick 

 deposits are laid down, the rise of the isogeotherms into them must 

 needs cause their expansion ; coupled with the observation of 

 American geologists, that in general a mountain- range occupies an 

 area where sedimentation has gone on undisturbed, until the deposit 

 has attained a great thickness, and that the event, next in sequence 

 there, has been the elevation of that very tract into a mountain- 

 range. The Americans have attributed this subsequent elevation to 

 the contraction of the Earth's volume through secular cooling, 

 acting upon the newly-deposited matter as a weak place. But 

 Mr. Eeade rejects this supposition. In this rejection I agree with 

 him. I think the question may be put simply thus. If any mass 

 contracts equably in all its parts, the rate of shortening of every 

 dimension must be the same. If then the whole Earth had 

 cooled down to the present temperature of the surface, every 

 dimension would have contracted equably, and there would be 

 formed neither wrinkles nor cracks. But in fact the crust has 

 cooled more than the interior. Therefore the circumference of the 

 crust must have contracted in a greater ratio than the radius of the 

 nucleus, which it envelopes. Its tendency would therefore be to 

 crack ; and, although the weight of its parts might close up the 

 cracks as they tended to be formed, still no compression would take 

 place, unless we throw more than its proper share of cubical 

 contraction into the vertical dimension.^ It does not seem that this 

 conclusion will be much affected by the condition of the interior, 

 whether it be solid or liquid.'* 



Accepting the theory of a solid globe, and the view, that the 

 matter of which it consists becomes fluid on the pressure being 

 reduced, the author thinks that this matter is subject to local 

 fluctuations of temperature, a property which it is not easy to 

 reconcile with solidity. Eocks at a depth of two miles and over, 

 whether originally soft mud or clay, may be treated as solid matter. 

 " Solid by compression, but ready to flow, one way or other, as the 

 pressure may be reduced or increased"' — a very accommodating 

 condition, but difficult to understand, because, if solid only by 

 compression, one would imagine that increased pressure would make 

 them more obstinate to move. 



There seems a difficulty in accounting for the accumulation of 



^ In a paper upon the subject, which I have lately contributed to the Phil. Mag. 

 (Feb. 1887), I have thrown the whole contraction into the vertical, a supposition, 

 as I have expressed it, " too highly favourable." 



* In considering the possible causes of contraction of the nucleus, perhaps enough 

 has not been made of the evisceration of the interior by volcanic action of various 

 kinds. How much must have been vomited forth to form the great basaltic flows — 

 and the pumice-covered floor of the deep oceans — and the dust clouds, which must 

 often and often in geological ages, for months and years at a time, have covered the 

 sky all round the globe ! ^ p_ gi_ 



