3i6 



NATURE 



[Fed. 1 8, 1875 



cal principles, and by our observation of what those forces have 

 effected in past times. 



" However we may differ in our interpretation of the present 

 thermal state of the globe, most geologists agree in accepting 

 the hypothesis of central heat as the one best in accordance whh 

 known facts relating to subterranean temperature, the eruption 

 of igneous rocks, the action of metamorphism, and the crushing 

 and conto rtions of rock masses. The radiation 'of heat into 

 space has been accompanied by a gradual contraction of the 

 central mass, and a shrmking of the crust, to virhich the trough 

 of oceans, the elevation of continents, the protrusion of mountain 

 chains, and the faulting of strata are to be attributed. The 

 question is whether that contraction was accompanied by a like 

 gradual yielding and adaptation of the solid crust to the lessening 

 circumference of the globe ; or whether the resistance of so rigid 

 a body was only overcome by paroxysmal efforts. This latter 

 vyas the view held by most of our early geologists, and is still 

 the prevailing one abroad. 



" It is not necessary to deal with the first steps of the problem. 

 Let us take it alter, lor example, the readjustment ot the crust, 

 when it must have been many miles thick, which resulted in the 

 elevation of such a mountain chain as that of the Alps ; and 

 here I must assume a point in advance. The resisting strata 

 having given way to the tension to which they had been sub- 

 jected, a state of equdibrium and repose would for a time ensue. 

 As the secular refrigeration subsequently proceeded, the tan- 

 gential force due to contraction resumed action, and while the 

 larger ar;as were depressed chiefly by the action of gravity, other 

 and smaller portions of the crust presenting the least resistance 

 yielded, and rose at right angles to the tangential pressure. 



" Now, either, if the elevatory force were limited and uniform in 

 degree, a point would be reached at which that force was ba- 

 lanced b) the increasing resistance and weight of the strata, and 

 the movement •would cease ; or else, if the energy was a con- 

 stantly generated quantity, and the rigidity such as to prevent 

 yielding beyond a certain extent (and no solid crust can be per- 

 fectly flexible), then it would be a dynamical necessity that a time 

 would come when, from the accumulation of that energy, it 

 would overcome the resistance, and the opposing strata be 

 suddenly rent and fractured. This primary resistance removed, 

 the full power of the elevatory force would be brought to bear 

 upon the di>jointed mass, and the surplus eneigy expended in at 

 once rapidly forcing forward and tilting up the now yielding 

 strata, along the line of fracture, to that position and that height 

 required to restore a state of equilibrium, and no more. It is 

 not pos>ible for *iy number 01 minor forces, where the ultimate 

 resistance exceeds each one taken separately, to accomplish in 

 any time, however long, that which requires for its execution a 

 major force of infinitely greater power. 



" Either a minor force, if sufficient to move a given weight, will 

 go on moving, or else, if from any cause a further or secondary 

 and independent resistance, such as, in this case, that dependent 

 on the cohesion of the strata, has to be overcome, additional pov/er 

 must be brought to bear, which, if that secondary resistance be 

 then overcome, the cumulated force being far in excess of the 

 residual resistance, will be immediately expended with energy in 

 proportion to the magnitude of the resistance mastered. 



" Agam, in the case of large faults traversing thick masses of 

 strata, the conditions are nearly the same. 



" I'he results of tlie foregoing conditions are in perfect accor- 

 dance with observation. The enormous crumpling and folding 

 of the strata — the vast upthrow of their disjointed edges — indi- 

 cate the resistless forces which have been at work. Of these 

 forces it is as difficult fjr us to realise the intensity as it is to 

 fathom the immensity of space. 



" While thus refrigeration progressed and the shell of the globe 

 became thicker, other causes came into operation to give it 

 greater rigidity, and so belter fit it lor the habitation of man. 



"In the many discussions to which this question hasigiven 

 rise, it has been too much assumed that the shell was of 

 uniform or nearly uniform thickness ; the irregularities of the 

 upper surface were apparent, but those possible on the under 

 surface have been overlooked. I have, however, reason to 

 suppose from some researches in which I have been engaged, 

 that the under surface of the shell is ribbed and channelled in a 

 manner and on a scale materially to influence the operation of 

 that tidal action on whicu so many able and elaborate calcula- 

 tions have been based. 



' ' Let us take on a continental area, having a mean surface tem- 

 perature of 55° F. , a point in the earth's crust through which any 

 isotherm of depth passes, - suppose it to be that of 1,000°. This 



earth-isotherm will possibly be found about a depth of about 

 50,000 feet. The isothermal plane must approximatively follow 

 the contours of the surface, and in mountain districts may rise 

 some 1,000 to 4,000 feet above its other level." 



Mr. Prestwich then shows that to the depth of the ocean we 

 have to add a depth equivalent to the difference between the 

 mean temperature of the adjacent land and that of the deep 

 waters. 



"As the position of the other earth-isotherms will in like 

 manner occupy successive planes approximatively parallel with 

 the surface whether of land or sea-bed, it follows that, if a central 

 molten nucleus exists, it will be divided into areas separated by 

 boundary lines, no less important than those formed by the con- 

 tinental areas between the several oceanic areas on the surface ; 

 and as they are even more enclosed and isolated, their condi- 

 tion with regard to the possible existence of tides would approach 

 more to that of an inland sea such as the Mediterranean, where 

 their influence is scarcely felt. It may be a question also whether 

 the rigidity of the earth's crust is not influenced by this mode of 

 structure. It must certainly affect the permanence of conti- 

 nental and oceanic areas. 



" Notwithstanding this, it may naturally be asked in view of the 

 more constant slow changes and movements to which, in past 

 times, the crust of the earth has been subject, and that even up 

 to a period so geologically recent as the elevation of the Alps 

 and the Andes, how it happens that it is now so quiescent and 

 comparatively immovable." Mr. Prestwich showed that the 

 hypotheses both of Mr. Hopkins and Sir W. Thomson grapple 

 with this difficulty, and in the same connection refers to the 

 theories of Mr. Mallet. Mr. Prestwich is not, however, satisfied 

 with the conditions suggested by these distinguished physicists, 

 and is led to seek for other causes to account for the present 

 stable condition of the earth. 



"The causewhich suggests itself tome," he said, " is the intense 

 cold of the glacial period through which the earth has so recently 

 passed, and which has, as it were, anticipated the refrigeration 

 which, in ordinary course, would have taken a longer time to 

 effect. At present the annual variation of temperature in these 

 latitudes extends to a depth of about 30 feet ; the maximum heat 

 of summer being felt by the end of November, and the maximum 

 cold of winter by the beginning of June at a depth of 26 feet. 

 But supposing the cold of winter not to alternate with summer 

 heat, then the abstraction of heat would continue to a depth 

 in proportion to the length of time during which the cold 

 at the surface was maintained and the degree of that cold, 

 and such would be the effect over a large portion of the 

 northern hemisphere (and I believe of the southern contem- 

 poraneously) during the glacial period. For as permanent ice 

 and snow then extended down to these latitudes, the summer 

 sun would not sensibly affect surfaces so covered, and the 

 abstraction of heat must have proceeded uninterruptedly. To 

 what depth the effect may have extended has not yet been inves- 

 tigated, but that it must have been very considerable is evident 

 from the depth to which the annual variations are now felt. 

 Consequently, with a uniform permanent temperature of 32", 

 or lower, at the surface, and with the long duration of the glacial 

 period, we may form some conception of how far beneath the 

 surface the extreme cold must have extended ; even "how, in 

 parts of Siberia, the ground is permanently frozen to a depth of 

 300 to 40Q feet. Then the surface temperature in these latitudes, 

 instead of commencing as now with a mean of 50^) and attaining 

 70' at a depth of 1,000 feet, commenced with a temperature of 

 32" F. or less, and the isothermal of 70° must have been de- 

 pressed far below its present level. On the return of the present 

 more temperate climate, that portion of the crust of the earth, 

 measuring certainly many hundreds, and possibly some thousands 

 ol feet in depth, which had suffered from this abnormal loss of 

 heat, would have to recover its equilibrium with existing condi- 

 tions by another change in the isothermal planes, and, until 

 that was effected, little or no loss by radiation would take 

 place. 



" Or, to look at it in another way, let us suppose periods of equal 

 temperature before and after the glacial epoch. As the radiation 

 of heat is in proportion to the difference of temperature between 

 the warm body and the surrounding medium, the loss of heat by 

 the earth would, if no colder period had intervened, have been 

 nearly equal in equal times ; but with the greater cold of the 

 glacial epoch, the same result would be effected in a shorter 

 time ; or, what is tantamount, the loss in the same time during 

 the glacial period would be greater than in the other two periods. 

 Thus, supposing we take any given time of the glacial period as 



