CXV111 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Mr. Hennessy more calculated to produce symmetrical relations in 

 the lines of fracture and elevation of that crust, than the squeezing 

 or crushing actions that would accompany a series of great subsi- 

 dences. The obvious connexion between the disturbances of the 

 crust of the earth and its thickness has produced some remarks on 

 that point. If gravity were rigorously perpendicular to the outer 

 surface of the earth's crust, its thickness might be extremely small 

 compared to the earth's radius, or the earth might be solid from its 

 surface to its centre : the manner in which Professor Stokes has 

 deduced Clairault's theorem shows, as remarked by Mr. Hennessy, 

 that the variation of gravity over the earth's surface, and other great 

 statical and dynamical results of the earth's structure, would be 

 then precisely the same, whatever might be its internal constitution. 

 But although gravity acts perpendicularly to the surface of the 

 ocean, that does not prove its perpendicularity to the earth's solid 

 crust; and reasons are adduced for coming to a different con- 

 clusion; whence Mr. Hennessy infers that we cannot at present 

 consider the thickness of the earth's crust as an inappreciable fraction 

 of the earth's radius. On the other hand, he considers Mr. Hopkins's 

 estimate of the thickness of the earth's crust as not conclusive, because 

 it essentially depends upon the hypothesis that the process of soli- 

 dification of the earth was accompanied by no change in the distribu- 

 tion of its particles. 



The very clear explanation of the views intended to be expressed in 

 his late essays, with which I have been favoured by Mr. Hennessy, has 

 enabled me to put other physical geologists in a position to estimate 

 their value ; and I have thus done justice to one who is following 

 closely and with great ability in the footsteps of Mr. Hopkins. The 

 purely physical investigation of the phenomena of the earth's crust 

 cannot be in better hands than those of our former president and of 

 his two Irish coadjutors Hennessy and Haughton, whilst there are 

 many of our members who are quite, capable of following Dr. Daubeny 

 in the purely chemical investigation. 



Amongst the phenomena which still require much elucidation are 

 those of metamorphism ; and it appears to me that we are at present 

 allowing our foreign friends to make a monopoly of that class of 

 research. M. Delesse has, for example, distinguished himself by his 

 efforts to clear up the difficulties which hang over the subject of 

 metamorphism ; and I shall therefore briefly notice his ' Etudes sur 

 le Metamorphisme ' here, as a preparative for an abstract of his 

 papers, which he has transmitted to me, and a translation of which 

 I propose to insert in a future number of the i Journal.' 



M. Delesse rightly observes that the term " metamorphism," taken 

 in its more general acceptation, comprises all the changes through 

 which rocks have passed ; and it may indeed be said that scarcely any 

 rock or stratum can be studied in its original condition, as every one, 

 whether it be a sandstone, a conglomerate, a shale, a schist, a 

 limestone, a trap, or a granite, has undergone some modification, — 

 consolidation being a process subsequent to deposition, and crystal- 

 lization subsequent to some form of liquefaction. He then introduces 



