GEOLOGY. 295 



the earth, that the centrifugal force, which gives the earth its spheroidal form, 

 in changing the form of the earth to correspond with the new position of the 

 poles, is sufficient to account for all the geological phenomena of the fracturing 

 of strata, elevation of mountain chains, etc. 



Dr. C. Y. Jackson remarked that if the sun and moon exerted a tidal action 

 upon the fluid matters of the interior of the globe as they do upon the ocean 

 waters, that it ought to be manifested by the rising and falling of the liquid 

 lavas of volcanoes, especially in those great volcanic openings in the Sandwich 

 Islands. He would ask Dr. Pickering, who was familiar with these volcanoes, 

 whether, at Kilauea, or at any of the other craters in those volcanic islands, 

 any regular periodicity was observable in the rising and falling of the liquid 

 lavas, and if so, whether they correspond to the times of the moon's phases ? 



Dr. Pickering replied that he was not aware of any regular periods of ele- 

 vation and subsidence of these lavas. He was under the impression that they 

 were quite irregular. 



Professor "W. B. Rogers remarked that while there was much ingenuity in 

 the idea of thus converting the insular volcanic mountain into a vast tide 

 gauge for measuring the movements of the fluid nucleus of the globe, we have 

 no right to anticipate any obvious correspondence between the fluctuations of 

 level in the liquid of the crater and the tidal movements beneath the earth's 

 crust. Supposing a connection to exist, the channels must be variable and 

 tortuous, and often probably connected with cavities containing gas and vapor, 

 and having numerous and changing outlets. From this would arise great and 

 variable resistances, retarding, diverting, and even arresting the movement 

 propagated from below, just as when the ocean tides are transmitted through 

 narrow and ramifying passages, or when they reach open spaces through 

 different channels and in opposite phases, we find the tidal phenomena greatly 

 modified, and sometimes even entirely destroyed. 



Recurring to M. Perrey's researches, Professor Rogers remarked that the 

 results, if confirmed by a fuller indication, would be of the utmost importance 

 to geological theory : first, by setting at rest any doubts that may exist as to 

 the igneous fluidity of the interior of the globe ; and secondly, by demonstrat- 

 ing the great thinness of the earth's crust, which alone could make it sensitive 

 to the tidal movements of the molten mass beneath. Such tenuity of the 

 crust had long since been urged by Professor H. D. Rogers and himself, as in- 

 ferable from the arched and folded structure of mountain chains, as well as 

 from the wave-like motion in earthquakes ; but geologists are still far from 

 being agreed on this and the allied points relating to internal heat. Even 

 the general fact of an increasing temperature as we descend below the sur- 

 face, although leading directly to the inference of an intense heat within 

 the earth, has not been accepted by all as proving the existence of & fluid 

 nucleus, and among those who admit the latter conclusion as demonstrated, 

 there are many who contend that the solid crust, instead of being some 30 

 miles, can not be less than 800 miles in thickness. This last estimate of the 

 thickness of the earth's crust, deduced by Professor Hopkins, of Cambridge, 

 England, from mathematical considerations connected with the precession of 

 the equinoxes, had apparently been accepted by Sir Charles Lyell as a basis 



