450 REID— ISOSTASY AND MOUNTAIN RANGES. [April 21. 



bottoms. Sir George Darwin'' has suggested that the continental 

 areas of the earth may be due to elevations caused by the differen- 

 tial retarding effect of lunar tidal action. But the theory of isostasy 

 tells us that they could not have maintained themselves unless they 

 were especially light ; and in this case they would have existed inde- 

 pendently of the tidal forces. Although these elevations, or 

 " wrinkles," as Sir George Darwin calls them, might have been dis- 

 torted by the dift'erent tidal effects in different latitudes, their orig- 

 inal meridional direction still requires explanation. 



The foldings and contortions of the rock have been so intimately 

 associated, in the minds of geologists, with mountain ranges, that 

 a low-lying region of folded rock has been looked upon as the 

 remains of a mountain range removed by erosion ; but as mountains 

 are not due to rock folding, this inference may be entirely wrong. 



Only a few of the consequences of the theory of isostasy have 

 been mentioned ; but the principle is of such fundamental importance 

 that it will surely exercise a strong influence over our future theories, 

 and will be applicable in directions not now suspected. Unfortu- 

 nately, it does not tell us definitely what is the cause of the elevation 

 of mountains and plateaux; but it limits our inquiries by excluding 

 all theories which assume the addition of matter to a segment. It 

 tells us, quite definitely, that the elevation of mountains, or the 

 depression of the oceans, must be due to vertical forces brought 

 about by a decrease, or increase, in the density of the material under 

 these regions. According to it, the mountains are high because their 

 material is light ; and as geological history tells us that the mountains 

 have not always existed, we must conclude that they were elevated 

 by an expansion of the material in and under them. And the great 

 deeps of the oceans are deep because the material under them is 

 dense and they have become deep by an increase in the density of 

 this material. Since all mountain areas are being lowered by active 

 erosion and many of the great ocean deeps are being filled by depo- 

 sitions, the great heights of the former must be due to the fact that 

 they are still in the process of elevation or that they have only 



""Problems connected with tlie Tides of a Viscons Spheroid," /'/;/'/. 

 Trans. R. S., 1879, Vol. 170, p. 589. 



