726 



REPORT — 1900. 



consequent on its loss of Leat, causes tlie crust to fall upon it in folds, which rise 

 over the continents and sink under the oceans, and the flexure ot the area of 

 sediiuentatiiin is partly a consequence of this foldinp, partly of overloading. By 

 the time a depression ot 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. tbe 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 tlie most below, that failure of 

 support can lead to sudden fracture and collapse; hence the comparatively super- 

 ficial origin of earthquakes. 



Given a sufficiently large coeflicient of expansion — and there is much 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 borderinc ocean we may be al)le to predict when and wliere 

 a mnunlain chdn 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 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 ofi'er a comparison of his results with those that follow from our 

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



Table slioiciixj tlie Time that has elapsed since the Beginning of the Systems in the 

 flrst column, as reckoned from Thickness of Sediment in tlie second column, 

 and by Professor Blytt in the third : — 



It is now time to return to the task, too long postponed, of discussing the data 

 from which we have been led to conclude that a probable rate at which sediments 

 have accumulated in places where they attain their maximum thickness is one foot 

 per century. 



Hale of Dejjosition of Sediment. 



_ We owe to Sir Archibald Geikie a most instructive method of entimating the 

 existing rate at which our continents and islands are being washed into the sea by 

 the action of rain and rivers: by this we find that the present land surface isbeino- 

 reduced in he'ght to the extent on an average of ^jij^ foot yearly.- If the 

 material removed from the land were uniformly distributed over an area equal to 

 that from which it had been derived, it would form a layer of rock ^^^tt foot thick 

 yearly, i.e. the rates of denudation and deposition wouldbe identical" But the two 

 areas, that of denudation and that of deposition, are seldom or never equal, the latter 



Vide p. 7X5. 



' According to Professor Penck 



3650 foot. 



