September 13, 1900] 



NATURE 



487 



I 



horizon and upon the stage in Keweenaw time, when the tilting 

 and erosion, which brought them to the surface, commenced. 



♦' That the syncHnal trough of Lake Superior began to form 

 before the end of the Keweenaw period, and consequently that 

 the Penokee rocks were not buried under the full succession, is 

 more than probable. However, they must have been buried to 

 a great depth — at least several miles — and thus subjected to high 

 pressure and temperature, notwithstanding which they are 

 comparatively unaltered" {Tenth Annual Report U.S. Geoi. 

 Survey, 1888-89, P- 457)- 



I select this example because it is one of the best instances of 

 a difficulty that occurs more than once in considering the history 

 of sedimentary rocks. On the supposition that the rate of 

 increment of temperature with descent is 1° F. for every 84 

 feet, or 1° C. for every 150 feet, and that it was no greater 

 during these early Penokee times, then at a depth of 50,000 

 feet the Penokee rocks would attain a temperature of nearly 

 333° C. ; and since water begins to exert powerful chemical 

 action at 180° C. they should, on the theory of a solid cooling 

 globe, have suffered a metamorphosis sufficient to obscure thefr 

 resemblance to sedimentary rocks. Either then the accepted 

 rate of downward increase of temperature is erroneous, or the 

 Penokee rocks were never depressed, in the place where they 

 are exposed to observation, to a depth of 50,000 feet. Let us 

 consider each alternative, and in the first place let us apply the 

 rate of temperature increment determined by Prof. Agassiz in 

 this very Lake Superior district : it is l° C. for every 402 feet, 

 and twenty-five millions of years ago, or about the time when 

 we may suppose the Penokee rocks were being formed, it would 

 be 1° C. for every 305 5 feet, with a resulting temperature at a 

 depth of 50,000 feet of 163" C. only. Thus the admission of a 

 very low rate of temperature increment would meet the diffi- 

 culty ; but on the other hand it would involve a period of several 

 hundreds of millions of years for the age of the " consist entior 

 status," and thus greatly exceed Prof, joly's maximum estimate 

 of the age of the oceans. We may therefore turn to the second 

 alternative. As regards this it is by no means certain that the 

 exposed portion of the Penokee series ever was depressed 

 50,000 feet : the beds lie in a synclinal the base of which indeed 

 may have sunk to this extent, and entered a region of metamor- 

 phosis ; but the only part of the system that lies exposed to 

 view is the upturned margin of the synclinal, and as to this it 

 would seem impossible to make any positive assertion as to the 

 depth to which it may or may not have been depressed. To 

 keep an open mind on the question seems our only course for 

 the present, but difficulties like this offer a promising field for 

 investigation. 



The Formation of Mountain Ranges. 



It is frequently alleged that mountain chains cannot be ex- 

 plained on the hypothesis of a solid earth cooling under the 

 conditions and for the period we have supposed. This is a 

 question well worthy of consideration, and we may first en- 

 deavour to picture to ourselves the conditions under which 

 mountain chains arise. The floor of the ocean lies at an average 

 depth of 2000 fathoms below the land, and is maintained at a 

 constant temperature, closely approaching 0° C. , by the passage 

 over it of cold water creeping from the polar regions. The 

 average temperature of the surface of the land is above zero, 

 but we can afford to disregard the difference in temperature 

 between it and the ocean floor, and may take them both at zero. 

 Consider next the increase of temperature with descent, which 

 occurs beneath the continents : at a depth of 13,000 feet, or at 

 same depth as the ocean floor, a temperature of 87° C. will lie 

 reached on the supposition that the rate of increase is l° C. for 

 150 feet, while with the usually accepted rate of 1° C. for 108 

 feet it would be 120° C. But at this depth the ocean floor, 

 which is on the same spherical surface, is at 0° C. Thus surfaces 

 of equal temperature within the earth's crust will not be spherical, 

 but will rise or fall beneath an imaginary spherical or spheroidal 

 surface according as they occur beneath the continents or the 

 oceans. No doubt at some depth within the earth the departure 

 of isothermal surfaces from a spheroidal form will disappear ; 

 but considering the great breadth both of continents ond 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 rocks still retain a high 

 temperature, and are probably not far removed from the critical 

 fusion point. The effect will be to render the continents mobile 

 as regards the ocean floor ; or, vice-versd, the ocean floor will 



NO. 161 I, VOL. 62] 



be stable compared with the continental masses. Next it may 

 be observed that the continents 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 in- 

 creases ; 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 sub- 

 siding area in the direction of least resistance, 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 geologists more cause for painful thought than 

 probably any other branch of their subject, not excluding 

 even the age of the earth. It was at first imagined that during 

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

 suffered so much contraction 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 pres- 

 sure 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 different 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 

 int, under the tremendous pressure of the depths of the crust, 

 b 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. O. 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 afford the ex- 

 planation we seek. The course of events may then proceed 

 as follows. The contraction of the interior of the earth, con- 

 sequent on its loss of heat, causes the crust to fall upon it in 

 folds, which rise over the continents and sink under the oceans, 

 and the flexure of the area of sedinientation is partly a conse- 

 quence of this folding, partly of overloading. By the time a 

 depression of some 30,000 or 40,000 feet has occurred along the 

 ocean border the relation between continents and oceans has 

 become unstable, and readjustment takes place, probably by a 

 giving way of the continents, and chiefly along the zone of 

 greatest weakness, i.e. the area of sedimentation, which thus 

 becomes the zone of mountain building. It may be observed 

 that at great depths readjustment will be produced by a slow 

 flowing of solid rock, and it is only comparatively near the 

 surface, five or ten miles at the most below, that failure of 



1 With some exceptions, notably Mr. C. Davison, a consistent supporter 

 of the theory of contraction. 



Z 



