Fisher — Cause of Changes of Level in the Earth's Crust. 217 



column and in the whole region affected, which will carry on 

 the movement until any given level in it contains liquid cooler, 

 and therefore more dense, than the average of the general mass 

 at the same level. Gravity and friction will gradually arrest 

 the motion, and, on account of the greater density, the liquid 

 in the region affected will begin to descend. The cooling pro- 

 cess will in the meanwhile he taken up by an ascending column 

 elsewhere. If we had begun the cycle with a descending cur- 

 rent, the reasoning and final result would have been similar. 



The surface of the liquid above an ascending column will 

 be somewhat elevated, and above a descending one depressed ; 

 thus the whole mass of liquid will experience the alternate 

 elevation and depression of regions of its surface, and if there 

 were a cooled crust floating upon it, in areas sufficiently wide 

 to compensate for the rigidity, it would partake in the changes 

 of level of the surface of the substratum. 



If a limited coastal region were elevated the change of level 

 would be local and merely confined to that region, — the effect 

 on the actual sea level being small. But if the region of the 

 sea bottom were depressed the level of the surface would be 

 everywhere lowered, and the coastal regions all over the globe 

 would apparently be raised ; and vice versa. The phenomena 

 seem to point to this depression or elevation of extensive 

 regions of the sea bottom as being the cause of the simul- 

 taneous recession from or approach to the coasts of the sea 

 margins. 



There appears to me no possible way of accounting for 

 the change of level indicated, unless upon the hypothesis of a 

 liquid substratum beneath the cooled crust. I am pleased to 

 notice that Dr. Nansen seems to approve of this theory. 

 Accepting this view, we get the important consequence of 

 isostacy which seems to have been confirmed by observations, 

 and on that theory I have explained the anomalies of the 

 deflection of the plumb line in India, in a paper which the 

 Superintendent of the trigonometrical surveys has thought 

 worthy of being printed as an appendix to a future report. 



Thus we have a suggestion offered for the cause of the 

 changes of level of the earth's surface, which appears worthy 

 of consideration as a working hypothesis. If the substratum 

 is liquid, this kind of movement must necessarily take place in 

 it ; but whether it would be quantitatively competent to pro- 

 duce the observed phenomena cannot easily be decided. That 

 would apparently depend chiefly upon the depth of the liquid 

 involved in the process. 



The amount of the elevation or depression of the surface 

 of the lithosphere, whether continental or suboceanic, would 

 be approximately measureable from the sea surface. And if 

 an area partly continental and partly oceanic were affected, 



