TRANSACTIONS OF SECTION C. 726 



surfaces from a spheroidal form will disappear; but considering tlie great breadth 

 both of continents and oceans this depth must be considerable, possibly even forty 

 or fifty miles. Thus the sub-continental excess of temperature may make itself 

 felt in regions where the r'icks still retain a hifrh temperature, and are probably not 

 far removed from the critical fusion point. The effect will be to render the con- 

 tinents mobile as regards the ocean floor ; or, vice versa, the ocean floor will be stable 

 compared with the continental masses. Next it may be observed that the con- 

 tinents pass into the bed of the ocean by a somewhat rapid flexure, and that it is 

 over this area of flexure that the sediments denuded from the land are deposited. 

 Under its load of sediment the sea-floor sinks down, subsiding slowly, at about the 

 same rate as the thickness of sediment increases; and, whether as a consequence or 

 a cause, or both, the flexure marking the boundary of land and sea becomes more 

 pronounced. A compensating movement occurs within the earth's crust, and solid 

 material may flow from under the subsiding area in the direction of least resist- 

 ance, possibly towards the land. At length, when some thirty or forty thousand feet 

 of sediment have accumulated in a basin-like form, or, according to our reckoning, 

 after the lapse of three or four millions of years, the downward movement ceases, and 

 the mass of sediment is subjected to powerful lateral compression, which, bringing 

 its borders into closer proximity by some ten or thirty miles, causes it to rise in 

 great folds high into the air as a mountain chain. 



It is this last phase in the history of mountain making which has given geo- 

 logists more cause for painful thought than probably any other branch of their subject, 

 not excluding even the age o* the earth. It was at l:rst imagined that during the 

 flow of time the interior of the earth lost so much heat, and suffered so much con- 

 traction in consequence, that the exterior, in adapting itself to the shrunken body, 

 was compelled to fit it like a wrinkled garment. This theory, indeed, enjoyed a 

 happy existence till it fell into the hands of mathematicians, when it fared very 

 badly, and now lies in a pitiable condition neglected of its friends.^ 



For it seemed proved to demonstration that the contraction consequent on 

 cooling was wholly, even ridiculously, inadequate to explain the wrinkling. But 

 when we summon up courage to inquire into the data on which the mathematical 

 arguments are based, we find that they include several assumptions the truth of 

 which is by no means self-evident. Thus it has been assumed that the rate at 

 which the fusion point rises with increased pressure is constant, and follows the 

 same law as is deduced from experiments made under such pressures as we can 

 command in our laboratories down to the very centre of the earth, where the 

 pressures are of an altogether difl'erent order of magnitude ; so with a still more 

 important coefficient, that of expansion, our knowledge of this quantity is founded 

 on the behaviour of rocks heated under ordinary atmospheric pressure, and it is 

 assumed that the same coefficient as is thus obtained may be safely applied to 

 material which is kept solid, possibly near the critical point, under the tremendous 

 pressure of the depths of the crust. To this last assumption we owe the terrible 

 bogies that have been conjured out of ' the level of no strain.' The depth of this 

 as calculated by the -Rev. 0. Fisher is so trifling that it would be passed through 

 by all very deep mines. Mr. C. Davison, however, has shown that it will lie 

 considerably deeper, if the known increase of the coefficient of expansion with rise 

 of temperature be taken into account. It is possible, it is even likely, that the 

 coefficient of expansion becomes vastly greater when regions are entered, where 

 the rocks are compelled into the solid state by pressure. So little do we actually 

 know of the behaviour of rock under these conditions that the geologist would 

 seem to be left very much to his own devices ; but it would seem there is one 

 temptation he must resist — he may not take refuge in the hypothesis of a liquid 

 interior. 



We shall boldly assume that the contraction at some unknown depth in the 

 interior of the earth is sufficient to afibrd the explanation we seek. The course of 

 events may then proceed as follows. The contraction of the interior of the earth, 



' With some exceptions, notably Mr. C. Pavison, a consistent gupporter of the 

 theory of contraction. 



