18G7.] HtJNT — THE CHEMISTRY OF THE EARTH. 233 



accords with the conclusions already arrived at relative to the 

 solidity of that nucleus, with the geological relations of these 

 phenomena as I have elsewhere shown ; and also with the remark- 

 able mathematical and astronomical deductions of the late Mr. 

 Hopkins, of Cambridge, based upon the phenomena of precession 

 and nutation ; those of Archdeacon Pratt ; and those of Pro- 

 fessor Thompson on the theory of the tides ; all of which lead to 

 the same conclusion — namely, that the earth, if not solid to the 

 centre, must have a crust several hundred miles in thickness, 

 which would practically exclude it from any participation in the 

 plutonic phenomena of the earth's surface, except such as would 

 result from its high temperature communicated by conduction to 

 the sedimentary strata reposing upon it. 



The old question between the plutonists and the neptunists, 

 which divided the scientific world in the last generation, was, in 

 brief, this — whether fire or water had been the great agent in giv- 

 ing origin and form to the rocks of the earth's crust. While 

 some maintained the direct igneous origin of such rocks as gneiss, 

 mica-schist, and serpentine, and ascribed to fire the filling of 

 metallic veins, others — the nuptunial school — were disposed to 

 shut their eyes to the evidences of igneous action on the earth, 

 and even sought to derive all rocks from a primal aqueous magma. 

 In the light of the exposition which I have laid before you this 

 evening, we can, I think, render justice to both of these opposing 

 schools. We have seen how actions dependent on water and acid 

 solutions have operated on the primitive plutonic mass, and how 

 the resulting aqueous sediments, when deeply buried, come again 

 within the domain of fire, to be transformed into crystalline and 

 so-called plutonic or volcanic rocks. 



The scheme which I have endeavored to put before you in the 

 short time alloted, is, as I have endeavoured to show, in strict 

 conformity with known chemical laws and the facts of physical 

 and geological science. Did time permit, I would gladly have 

 attempted to demonstrate at greater length its adaptation to the 

 explanation of the origin of the various classes of rocks, of metallic 

 veins and deposits, of mineral springs, and of gaseous exhalations. 

 1 shall not, however, have failed in my object, if, in the hour 

 which we have spent together, I shall have succeeded in showing 

 that chemistry is able to throw a great light upon the history of the 

 formation of our globe, and to explain in a satisfactory manner 

 some of the most difficult problems of geology ; and I feel that 



