Local Attraction on Geodetic Operations, 378 



whereas at Kaliana, and stations still further off, the attraction of the 

 mountain-mass above the sea-level, and the deficiency of attraction from 

 the crust below that level, would nearly counterbalance each other. Thus, 

 if the thickness of the crust below the plains is 100 miles, and the amount 

 of matter in the crust under the plains equals that of the crust and moun- 

 tains together in the mountain-region, then the deflections at Kaliana, 

 Kalianpur, and Damargida, instead of being 27"-98, 12"*05, 6"'79, arising 

 from the mountains alone, are reduced to 1"'54, — 0"-06, — 0"-06 (see 

 Philosophical Transactions for 1858, p. 759), which are all insigni- 

 ficant compared with the large deflections caused by the mountains 

 alone. 



This theory, that the wide ocean has been collected on parts of the 

 earth's surface where hollows have been made by the contraction and 

 therefore increased density of the crust below, is well illustrated by the 

 existence of a whole hemisphere of water, of which New Zealand is the 

 pole, in stable equilibrium. "Were the crust beneath only of the same 

 density as that beneath the surrounding continents, the water would be 

 drawn off by attraction and not allowed to stand in the undisturbed posi- 

 tion it now occupies. 



17. I have, in what goes before, supposed that, in solidifying, the crust 

 contracts and grows denser, as this appears to be most natural, though, 

 after the solid mass is formed, it may either expand or contract, according 

 as an accession or diminution of heat may take place. If, however, in the 

 process of solidifying, the mass becomes lighter, the same conclusion will 

 follow — the mountains being formed by a greater degree of expansion of 

 the crust beneath them, and not by a less contraction, than in the other 

 parts of the crust. It may seem at first difficult to conceive how a crust 

 could be formed at all, if in the act of solidification it becomes heavier 

 than the fluid on which it rests ; for the equilibrium of the heavy crust 

 floating on a lighter fluid would be unstable, and the crust would sooner or 

 later be broken through, and would sink down into the fluid, which would 

 overflow it. If, however, this process went on perpetually, the descending 

 crust, which was originally formed by a loss of heat radiated from the 

 surface into space, would reduce the heat of the fluid into which it sank, 

 and after a time a thicker crust would be formed than before, and the 

 difficulty of its being broken through would become greater every time a 

 new one was formed. Perhaps the tremendous dislocation of stratified 

 rocks in huge masses with which a traveller in the mountains, especially 

 in the interior of the Himmalaya region, is familiar, may have been brought 

 about in this way. The catastrophes, too, which geology seems to teach 

 have at certain epochs destroyed whole species of living creatures, may 

 have been thus caused, at the same time breaking up the strata in which 

 those species had for ages before been deposited as the strata were formed. 

 These phenomena must now long have ceased to occur, at any rate on a 

 Tery extensive scale, as Mr. Hopkins's investigations on Precession appear 



