PHYSICS AND MATHEMATICS TO GEOLOGY. 149 
strong to waste upon it a collateral glance” (l. ¢, p. 259). Accordingly 
he crushes Sir W. Thomson’s arguinent* from the tides by the remark, 
“that a siliceous crust of 20 miles average thickness and an overlying 
aqueous ocean of 5 miles average depth, should have (as required by 
the argument) so equal a coefficient of mobility that sea and land could 
thus together ‘rise and fall,’ might well be pronounced incredible” 
(7. ¢., p. 260). 
He regards Sir W. Thomson as very seriously damaging his own ar- 
gument by the admission that tides comparable in magnitude with those 
observed would occur even in a solid earth of steel. It does not seem 
to have occurred to him that the existence of a difference between the 
motions of the land and water may constitute an argument for solidity.t 
Mr. Taylor admits one difficulty in his theory, viz, the nature and 
local characteristics of the plications actually observed, and remarks: 
“While the force at the command of the rotating planet is abundantly 
Sufficient - - - evidently some supplementary considerations are 
requisite to give the observed direction to this force,” - - - “The 
mere mechanical difficulty however of transmitting stresses through 
comparatively undisturbed areas of hundreds of miles of a flexible, fri- 
able, and practically plastic crust—with a large coefficient of viscous 
friction beneath—is not so formidable as might at first appear. It 
must be borne in mind that the pressures derived from an action so 
slow as from century to century to be scarcely sensible, are of an or- 
der of very great intensity, but of very small quantity” (l. ¢., p. 265). 
Mr. Taylor also infers from “ various considerations ” that “in all ages 
mountain building has been at amaximum; thatis, the uplifted heights 
have been the greatest which the average thickness of the crust at the 
time was capable of supporting; so that the former has been a constant 
function of the latter, the ratio being probably not far from one-fifth” 
(l. ¢., p. 265). Mr. Taylor does not state that this law of the uplifted 
heights is true of all lands as wellas of all time, but the possibility that 
such may be the case is rather alarming. He enters in fact into no un- 
necessary details as to how he reached his conclusions, so that all one 
can say is that, measured by his own standard, he is certainly not in- 
ferior in physical insight even to Mr. Herbert Spencer. Perhaps when 
he comes to deal with the ** supplementary considerations” he may sup- 
ply sufficient data for the mathematician to follow him. 
Prof. Prestwich, in his Geology, vol. U, regards the “ present very 
great rigidity of the earth” as being proved by mathematical 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 
escit or a astic; nor is if necessary that this magma should be of any 
- Natural Philosophy, Vol. 1, See Il, 1, § 833. 
t See his remarks, /. c., p. 260, and foot-note. 
