460 Notices of Memoirs — Professor Sottas' s Address 



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 



We shall boldly assume that the contraction at some unknown 

 depth in the interior of the earth is sufficient to afford the explana- 

 tion we seek. The course of events may then proceed as follows. 

 The contraction of the interior of the earth, consequent 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 

 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 support can lead to sudden 

 fracture and collapse ; hence the comparatively superficial origin of 

 earthquakes. 



Given a sufficiently large coefficient of expansion — and there is 

 miuch to suggest its existence — and all the phenomena of mountain 

 ranges become explicable : they begin to present an appearance that 

 invites 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 

 worldwide 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 regard to the estimated duration 

 of geological time. 



Influence of Variations in the Eccentricity of the Earth's Orhit. 

 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 discussion of Professor Blytt's methods, 

 we may offer a comparison of his results with those that follow from 

 our rough estimate of one foot of sediment accumulated in a century. 



Table showing the time tliat has elapsed since the Beginning of the Sijstems in 

 the first column, as reckoned from Thickness of Sediment in the second 

 column, ayid by Professor Blytt in the third: — 



^ocene 4,200,000 3,250,000 



Ohgocene 3,000,000 



^iiocene 1,800,000 



Pliocene 900,000 



Pleistocene 400 000 



1,810,000 



1,160,000 



700,000 



350,000 



