484 OSBORN AND REEDS PREHISTORY OF MAN IN EUROPE 



elusion, we shall do well to follow Gilbert's advice and "go behind the 

 postulates" ; especially behind the postulates that a considerable stretch 

 of a continental coast can not rise equably, and that terraces separated 

 by similar vertical intervals in the valleys of different rivers at a con- 

 siderable distance inland from the river mouths indicate eustatic changes 

 of ocean level, De Lamothe's treatment of the river-terrace half of his 

 problem is far from satisfactory, because it seems tacitly to assume that 

 the fall or grade of a river is constant during successive epochs of aggrad- 

 ing and degrading its valley, and this is altogether improbable. 



As to the coastal half of the problem, no thorough and competent 

 inquiry has yet been made by de Lamothe or any one else, No one has 

 yet determined how large a fraction of the entire length of all conti- 

 nental and insular coastlines ought to exhibit shoreline terraces at given 

 heights above sealevel standard, in the sense that each terrace represent- 

 everywhere the same time measure of abrasional or depositional work, 

 allowance being made for nature of coastal rocks, outline of coast, and 

 exposure to wave action where terraces are cut, and for area drained 

 where delta terraces are built, in order to warrant the conclusion that 

 coasts thus terraced have stood still and the ocean level has changed. 

 It is easy to conceive that if a part of the ocean floor first rose and then 

 sank while all the coasts and islands of the world stood still, a temporary 

 high-level shoreline would now constitute a coastal terrace everywhere 

 at a uniform altitude above present sealevel, and that all river valleys 

 should show appropriate effects of stream-filling and erosion. It is also 

 easy to conceive that if the disturbed ocean-floor area which rose and 

 sank were extended to include part of a continental border, then that 

 part of the continental border would not be terraced like the rest of the 

 coasts of the world. 



It is. furthermore, easy to imagine that if some such fraction as nine- 

 tenths of the coasts of the world exhibited a standard terrace everywhere 

 at the same altitude (the corresponding features of river valleys are here 

 left unmentioned, to save space), while the remaining tenth exhibited 

 no such terrace, then the ocean and the non-terraced coastal tenth must 

 have changed while the nine-tenths remained fixed. But it is absolutely 

 unwarranted in the present state of exploration of the world's coasts — 

 one might almost say in the present state of mental equipment for such 

 exploration — to assert that a large share of the world's coasts possesses 

 a standard terrace; and until such an assertion can be safely made, the 

 assumption of eustatic instead of epirogenic change is unjustified. For 

 example, if such an assertion be based on the occurrence of a terrace 

 along several hundred miles or even along one or two thousand miles 



