Physics and Mathematics to Geology. 349 



"present very great rigidity of the earth "as proved by mathe- 

 matical and physical investigations, but complains of a " want 

 of elasticity " in the methods of the mathematicians (p. 538). 

 According to him, " The hypothesis most compatible with the 

 geological phenomena is that of a central solid nucleus with 

 a molten yielding envelope — not fluid, but viscid or plastic; nor 

 is it necessary that this magma should be of any great thick- 

 ness ; but a thin crust is, it seems to me, an essential con- 

 dition'''' (p. 543). Professor Prest-wich adduces in support of 

 his views various arguments from geological phenomena which 

 seem of much weight. He has also various arguments of a 

 more or less physical character, but they seem to take a good 

 deal for granted. Thus, on p. 540, referring to plications in 

 the surface rocks, he says, " if the earth were solid through- 

 out, the tangential pressure would result not in distorting or 

 crumpling, but in crushing and breaking. As a rule no such 

 results are to be seen, and the strata have . . . yielded, as only 

 a free surface-plate could, to the deformation caused by lateral 

 pressure ... a yielding bed, on which the crust could move 

 as a separate body, was necessary ." It seems to me that as 

 the phenomena of rupture are as yet very imperfectly ascer- 

 tained, except perhaps for a few simple standard conditions, 

 Professor Prestwich has very little to go on but a priori ideas. 

 I fail to see, for instance, why pressures at or near the surface 

 of a solid sphere should necessarily produce fracture and not 

 flow. Also it seems improbable that there would be a sharp 

 line of demarcation so as to enable a crust — which seems 

 clearly to mean a solid superficial layer — to move as a separate 

 body on a " yielding bed." Would not this imply a liquid 

 substratum with no appreciable viscosity ? And supposing 

 there were a substratum of this kind, is there any sufficient 

 experimental evidence that a solid crust of even a few miles 

 thickness would on the falling away of the liquid underneath 

 go into folds instead of being crushed and broken? Further, 

 can plications, to the extent shown, say by the Alps, be re- 

 conciled with the retention of contemporaneous solidity? 

 Supposing the earth to be essentially solid throughout, is there 

 any reason why the strain at some miles below ihe surface 

 should not locally at intervals exceed the elastic limit, with 

 the result for a time of a state of flow or plasticity throughout 

 a volume of greater or less extent? During such an epoch 

 there would exist locally conditions somewhat resembling those 

 which Professor Prestwich believes existent everywhere. It 

 is true that one argument adduced by Professor Prestwich and 

 others against the existence of separate reservoirs of molten ma- 

 terial — viz. the similarity in the character of volcanic products 



