Reviews — W. Carruthers on the Fossil CycadecB. 573 



during the night, and during summer than during winter, proves that 

 there must be some physical connexion between the heat of the sun 

 and the motion of the glacier. Mr. CroU could never harmonize 

 this fact with the commonly-received theory ; and, while admitting 

 the agency of heat, he remarks that heat is not necessarily the cause 

 of glacier-motion. Gravitation may be the cause, and heat only a 

 necessary condition. 



To him there seems to be but one explanation, namely, that the 

 motion of the glacier is molecular. His concluding remarks are as 

 follow : The ice descends molecule by molecule. The ice of a glacier 

 is in the hard, crystalline state, but it does not descend in this state. 

 Gravitation is a constantly acting force ; if a particle of the ice lose 

 its shearing-force, though but for the moment, it will descend by its 

 weight alone. But a particle of the ice will lose its shearing-force 

 for a moment if the particle loses its crystalline state for the moment. 

 The passage of heat through ice, whether by conduction or by radia- 

 tion, in all probability is a molecular process, that is, the form of 

 energy termed heat is transmitted from molecule to molecule of the ice. 

 But at the moment that it is in possession of the passing energy, is 

 the molecule in the crystalline or icy state ? If we assume that it is 

 not, but that in becoming possessed of the energy it loses its crystal- 

 line form, and for the moment becomes water, all our difficulties 

 regarding the cause of the motion of glaciers are removed. We 

 know that the ice of a glacier, in the mass, cannot become possessed 

 of energy in the form of heat without becoming fluid ; may not the 

 same thing hold true of the ice-particle ? 



TMchness of Glaciers. — Appended to his paper is a note by Mr. 

 CroU on the alleged limit to the thickness of a glacier. Experiments 

 by Canon Moseley, on the crushing of ice, led this gentleman to 

 conclude, that if a glacier be over 710 feet in thickness, the ice at 

 the under surface must be crushed by the incumbent weight. Ex- 

 periments previously made by Professor Phillips had led him to the 

 conclusion that a glacier, exceeding 1,500 feet in thickness, would 

 lose its solidity. 



Mr. Croll maintains that, as a necessary consequence of the pro- 

 perty of regelation, ice, after being crushed, would resolidify. So 

 far as he is aware, there is no known limit to the amount of pressure 

 which ice may sustain. On the Antarctic continent we have reasons 

 for believing that the ice is in some places over a mile in thickness. 



laiEJ'VIE^WS. 



I. — On the Eossil Cvcadean Stems from the Seoondart Eocks 

 OF Britain. By William Carruthers, F.L.S., F.G.S., British 

 Museum. Transactions of the Linnean Society of London, Vol. 

 xxvi., 1870, pp. 675-708. 4to. 10 plates. 



OUR knowledge of the true classification of the remains of fossil 

 plants has greatly increased of late; not that any very large 

 additions have been made in the number of species described since 

 the days of Lindley and Hutton, but we have become better ac- 



VOL. VII. — NO. LXXVIII. 37 



