INTRODUCTION. ill 



with primeval granite*. In the other most ancient rocks, 

 particularly those of hornblende or siderite, iron also abounds; 

 as it does in jasper, common slate, trap, serpentine, and the 

 oldest sandstone. 



It may be necessary, however, to introduce a distinction 

 between the Siderous Earth and metallic iron, which must 

 depend upon the proportion to be found in various substances. 

 As the alkaline earths, though they yield metals, will scarcely 

 by any mineralogist be classed among metallic substances 5 so 

 Siderous Earth, though it yield iron, may be admitted among 

 the other principal substances of that class. In fact, as the Dr. Davy'* 

 recent discoveries of Dr. Davy evince that the alkaline earths, di 8 " 68 - 

 that is the calcareous, magnesian, barytic, strontianic, are of 

 a metallic nature or yield peculiar metals, while he suspects 

 the other earths to be in the like predicament (as the sili- 

 ceous has since proved), it would be absurd to reject iron as 

 an earth, merely because it yields a metal. 



It has already been seen that the presence of this earth, Power of iron, 

 even in a small quantity, is so powerful as greatly to affect 

 the compound ; and, by altering the quality of its mode, 

 even to change its substance and denomination. Among 

 many examples may be mentioned the hyalite of Dauphiny, 

 which only contains 10 of iron in 100, yet the form of the 

 crystals is that of the iron ore of Elba. Many mineral sub- 

 stances receive their denominations, not from the abundance, 

 but from the influence of particular earths ; for the greater 

 part of the argillaceous and talcous rocks contain two thirds 

 or three quarters of silexj so that if the abundance alone 

 were regarded, two of the chief denominations of the mi- 

 neral kingdom would be rejected. In many instances, the 

 energy of one ingredient has far more force than the abun- 

 dance of another. An able chemist, after discussing some Energy of 

 difficulties of this kind, thus proceeds : < < These circum- in re ients - 

 stances no doubt arise from the modes of chemical analysis 



* The granite of the Hartz even affects the Magnet. Jam, Miu. Sc. Isles, 

 p. 65. 



