1819.] Greenough on the First Principles of Geology. 463 



It would be too much to suppose that Werner's arrangement 

 of the fioetz rocks, which he himself admitted to be very imper- 

 fect, is rigidly accurate ; yet when we compare it with the 

 structure of England, the agreements are much greater than we 

 could have expected a priori. Some of Werner's fioetz forma- 

 tions, indeed, as his gypsums, either do not exist in England, or 

 they constitute beds of trifling size and extent ; while some, as 

 the oolite, occupy a much greater space in England than they 

 seem to do in Saxony and Bohemia, from which Werner seems 

 to have drawn his notions. But whoever makes a comparison 

 of the Wernerian fioetz rocks with Mr. Smith's geological map 

 of England, or with Mr. Buckland's table of the order of the 

 strata of England and Wales, will be struck with the resem- 

 blance. 



I do not see the force of our author's refutation of the 

 Wernerian assumption, that granite is the fundamental or lowest 

 rock. \ 



With respect to the specific gravity of rocks, though it is not, 

 strictly speaking, true, that the rocks which constitute the sur- 

 face of our earth are deposited accurately according to their 

 relative specific gravities ; yet when we allude to the structure 

 of the whole globe, it is obvious that the concentric shells of 

 which the globe is composed must increase in specific gravity 

 as they approach the centre. This would be the case even 

 supposing every shell composed of the same kind of rock. The 

 reason is, that every shell is subjected to the pressure of all the 

 shells between itself and the surface of the earth ; so that the 

 specific gravity of each shell must increase in a geometric ratio 

 as it approaches the centre of the earth. Let us suppose the 

 earth composed of a million of concentric shells, inclosed withia 

 each other like the coats of an onion. Let the specific gravity 7 

 of the outermost shell be 2-|, that of water being 1 ; then it has 

 been demonstrated that the specific gravity of the stratum form- 

 ing the millionth shell, or the nucleus of the globe, supposing it 

 merely increased by the pressure of the shells lying over it, 

 would exceed 13-i-. It is obvious from this that we cannot 

 reason about the nature of the strata which constitute the inte- 

 rior of the earth ; because what materials soever we suppose 

 them composed of, the specific gravity would increase geome- 

 trically as they approach the centre. 



The work, which I have been analyzing, contains three more 

 essays, with the following titles : 



VI. On the Properties of Rocks, us connected with their 

 respective Ages. 



VII. On the History of Strata, as deduced from their Fossil 

 Contents. 



VUI. On Mineral Veins. 



These essays contain much information, and many judicious 

 observations, in the justice of which I heartily -concur. But 1 



