[ 13 ] 



III. On the Structure of the Earth By a Correspon- 

 dent. 



To Mr. Tillock. 



Sib, J.N the present uncertainty v\hicli is extended over 

 geological scieiiCC, and the great diversity of opinion which 

 prevails even with regard to its first principles, it is not to 

 circuinstances of local particularity that our attention 

 should be directed so nujch as to facts universal in extent, 

 and which point towards a knowledge, however imperfect, 

 of the internal construction of our globe. It has indeed 

 been well rertuuked, that the inclination of its superticial 

 strata enables us to extend our views far beneath the limits of 

 human labour; yet, even taking this into the account, we 

 cannot but confess our inability to pierce beyond the sur- 

 face. Mathematicians have deternimed J '2 miles for the 

 mean depth of the ocean; while the continents, so far as 

 our knowledge extends, appear to consist of substances of 

 from 'i to 2^ times the density of water, and lo rest on 

 granitic masses whose density docs not exceed this limit. 

 The superficial density of our globe may therefore be sta'cd 

 at about \\. Yet the mean density is known to amount 

 to nearly five times that of water. The great mass there- 

 fore wliich composes the nucleus of the earth must have 

 about this density. With this fact appears intimately con- 

 nected a question of great imporlance on the formation of 

 mineral veins, w hence is derived the matter which occupies 

 them? A Neptunist will say, From above, by infiltration; 

 while those who maintain the Plutonic system answer, 

 From below, by injection, when fused. The arguments on 

 both sides have been handled with great ability ; but the 

 density of many of these extraneous masses affords one 

 which, though hitherto unnoticed, appears to me at once 

 to decide their origin. If it be from above, how happens 

 it that the source abovt, J rom u hence it introduced Uself, 

 has never in <jne single instance been discovered ? Per- 

 haps the supply is already expended; perhaps it has already 

 been carried down, with the rest of ihedci/is of the soil, to 

 the ocean. Still, however, iis existence is hvpotheticul. 

 On the other hand, the Huttonian explanation is subject to 

 no such embarrassing queries. The density of nuiallic 

 veins, deviating in so remarkable a manner from every sur- 

 rounding phaeni mjn(;n, directs us immediately to look for 

 their source in tiie great reservoir winch we know to con- 

 tain all the more solid parts of our i-lanet. II is evidently 



no 



