792 



SCIENCE. 



[N. S. Vol. XII. No. 308. 



from experiments made under such pres- 

 sures as we can command in our laboratories 

 down to the very center of the earth, where 

 the pressures are of an altogether differ- 

 ent order of magnitude ; so with a still 

 more important coefficient, that of expan- 

 sion, our knowledge of this quantity is 

 founded on the behavior of rocks heated 

 under ordinary atmospheric pressure, and 

 it is assuaied that the same coefficient as is 

 thus obtained may be safely applied to ma- 

 terial which is kept solid, possibly near the 

 critical point, under the tremendous pres- 

 sure of the depths of the crust. To this last 

 assumption we owe the terrible bogies that 

 Lave 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 ex- 

 pansion 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 behavior 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 must not take refuge in the 

 hypothesis of a liquid interior. 



We shall boldly assume that the contrac- 

 tion at some unknown depth in the interior 

 of the earth is sufficient to afford the expla- 

 nation we seek. The course of events may 

 then proceed as follows : The contraction 

 of the interior of the earth, conseqvient 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 sedimentation is partly a 

 consequence of this folding, partly of over- 

 loading. By the time a depression of some 



30,000 or 40,000 feet has occurred along the 

 ocean border the relation between conti- 

 nents and oceans has become unstable, and 

 readjustment takes place, probably by a 

 giving waj' of the continents, and chiefly 

 along the zone of greatest weakness — i. e., 

 the area of sedimentation, which thus be- 

 comes the zone of mountain building. It 

 may be observed that at great depths read- 

 justment 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 support can lead 

 to sudden fracture and collapse ; hence the 

 comparatively superficial origin of earth- 

 quakes. 



Given a sufficiently large coefficient of 

 expansion — and there is much to suggest 

 its existence — and all the phenomena of 

 mountain ranges become explicable ; they 

 began to present an appearance that in- 

 vites mathematical treatment ; they inspire 

 us with the hope that from a knowledge of 

 the height and dimensions of a continent 

 and its relations to the bordering ocean we 

 may be able to predict when and where 

 a mountain chain should arise, and the 

 theory which explains them promises to 

 guide us to an interpretation of those 

 world-wide unconformities which Suess can 

 only account for by a transgression of the 

 sea. Finally it relieves us of the difficulty 

 presented by mountain formation in re- 

 gard to the estimated duration of geological 

 time. 



INFLUENCE OF VARIATIONS IN THE ECOEN- 

 TEIOITT OF THE EARTH'S ORBIT. 



This may perhaps be the place to notice 

 a highly interesting speculation which we 

 owe to Professor Blytt, who has attempted 

 to establish a connection between periods 

 of readjustment of the earth's crust and 

 variations in the eccentricity of the earth's 

 orbit. Without entering into any discus- 

 sion of Professor Blytt's methods, we may 



