ANNIVERSARY ADDRESS OF THE PRESIDENT. CXVll 



proceeding from its surface to its centre, consequent on the overlying 

 strata compressing those included within them, would render the 

 process of "convection" very different from that which takes place in a 

 homogeneous and limpid fluid, such as warm water, and thus the prin- 

 cipal oscillations of the fluid would be confined to the vicinity of the 

 surface. As explanatory of the influence of the viscidity of the fluid, 

 the consolidation of the surface of lava-streams is adduced, as it shows 

 how a continuous solid covering may be gradually formed over matter 

 still continuing in a fused condition beneath. The probably imper- 

 fectly consolidated, porous, or scoriaceous condition of the first-soli- 

 dified portions of such a fluid mass would, as he has subsequently 

 remarked, greatly facilitate the formation of a solid crust ; and if the 

 first superficial pellicle of the earth's crust had been formed in this 

 way, it would manifestly be much more easily disintegrated and 

 removed than the harder and more slowly consolidated crystalline 

 masses beneath. 



In further considering the physical and mechanical actions by which 

 the earth's internal structure may have been effected, Mr. Hennessy 

 strongly objects to the adoption of the supposition, regarding the 

 fluid matter from which the crust of the earth has solidified, that 

 the change of consistence of the earth's materials produced no change 

 in their distribution. This hypothesis he rejects as improbable, and 

 considers it more philosophical to base our views regarding the 

 changes of state in the earth's materials upon the evidence afforded 

 by those materials which come under our actual notice. The nature 

 of the earth's internal structure appears to depend greatly upon the 

 manner in which its fluid portions have passed into a solid state ; and 

 Mr. Hennessy points out that a consequence deducible from his views 

 of the earth's structure, and from a result obtained by Mr. Hopkins, 

 would prove the existence of considerable pressure and friction be- 

 tween the fluid nucleus of the earth and its solid envelope. The 

 physical causes on which this pressure depends are shown to be 

 involved in the gradual increase of density of the fluid towards its 

 central portions. The connexion of these views with that portion of 

 geological dynamics which embraces the study of the greater eleva- 

 tory movements on the surface of our planet is briefly pointed out. 

 "Without implying the correctness of all the conclusions stated by M. 

 Elie de Beaumont, Mr. Hennessy is satisfied that the eminent French 

 geologist has shown such an amount of connexion between very 

 widely distributed phenomena of elevation as indicates the operation 

 of general and wide-spread disturbing agencies beneath the crust of 

 the earth. But, instead of attributing, with De Beaumont and 

 Humboldt, these phenomena to the successive collapse of the crust 

 of the earth upon a more rapidly contracting nucleus, Mr. Hennessy 

 refers it to the pressure he had before explained. Illustrating his 

 views from the equilibrium of arches, he shows that a pressure 

 acting in the direction required to produce subsidence of the earth's 

 crust upon the nucleus, would be far more resisted than a press- 

 ure acting in the contrary direction. The action of an interior 

 expanding elevatory force upon the crust of the earth appears to 



