Geology* 415 
Alumina lias the specific gravity 2, calcia, 2.3, mag- 
nesia 2.3, silica 2.65. The metallic ores contained in 
these formations, are too few in number and in quantity 
to raise the specific gravity of the mass ,1. Add to this,, 
that the quantity of sea & river water contained in these for- 
mations will greatly reduce the specific gravity of the mass ; 
so that the average specific gravity of all the known strata 
of the earth cannot fairly be considered as amounting to 
more than 2 ; but if taken at 2.5, then will the known 
strata possess a specific gravity of one half the specific 
gravity of the whole globe. Hence it will follow, that 
this earth consists of a nucleus, of a metallic nature, 
whose specific gravity exceeds' 5, covered by a crust con- 
sisting of a series of formations having together a speci- 
fic gravity not quite reaching 2.5. 
This outward crust, including the rivers and oceanp 
that rest upon and within it, seems to be the only object 
of examination to the Geologist, or as the German phi- 
losophers affect to say, the Geognost. No observation 
that I know of, has hitherto extended beyond the granite 
formation that appears as its substratum. I know of no 
volcanic ejection that will warrant us in concluding that the 
matter thrown out, is any other than part of the forma- 
tions that constitute this crust. The following questions 
admit but of conjectures. 
What are the constituent parts of the nucleus ? 
Is there any series of cavities between the nucleus and 
the crust ? 
What is the thickness of the crust ? 
Of how many original and universal strata, or forma - 
tions does this crust consist ? 
I tread upon new ground ; but I use the aids which 
wise men have furnished ; I have none of my own. 
The nucleus is metallic : I conclude tins, from its spe~ 
cific gravity ; far too great for any known earth. Sir Isaac 
