Geology. — Oji tJie Constituent Minerals, SfC. 387 



that all these earths are compounds of oxygen united with me- 

 tallic bases, so that the whole mineral volume of the planet, may 

 be considered as a mass of metals and their modifications. Ochre, 

 or ferruginous earth, is easily converted into iron, as it is con- 

 stantly done at the furnaces. It stands in the same relation to 

 that metal, which the rust found upon polished steel does, when 

 exposed to moisture ; both of them are oxides of iron. Quartz, 

 flint, sand, all substances which are purely silicious, can also, 

 but with greater difficulty, be converted into their metallic base, 

 silicum. The same may be asserted of lime and magnesia. 



Mineral bodies are purely simple, when they consist of one 

 kind of earth unmixed with any other ; and compound, when 

 their constituents are formed of more than one mineral substance. 

 Besides this variety in their composition, it is to be remarked, 

 that their structure is various, some being formed of separate 

 minute bodies of the same substance mechanically approximated 

 together, and others being an aggregate of irregular crystals, 

 produced by an apparent simultaneous crystallization. This 

 distinction is of the highest importance in relation to the dividing 

 line, of which we have spoken, between the primary and se- 

 condary rocks. 



Silica, is the basis of rock crystal, quartz, flint, chert, and 

 sandstone. The first of these substances presents us with silica 

 in its purest crystalline form ; the last is an aggregate of minute 

 bodies of silica mechanically approximated together. 



Alumina, is the basis of the sapphire ; but the best kind of 

 Fuller's earth, and the purest clays, are aggregates formed of 

 alumina. 



Ldme, is the basis of all calcareous bodies, but the best statu- 

 ary marble, as well as many sub-crystalline limestones, is an 

 aggregate of irregular crystals, produced by an apparent simul- 

 taneous crystallization. 



Magnesia is not frequently found pure, but it enters freely 

 into the composition of some of the primary rocks, as we shall 

 hereafter have to state, and occurs also in certain limestones. 



These earths, sometimes simple, sometimes in a compound 

 state, together with the alkalies, oxides of iron and manganese, 

 sulphur in combination with lime, carbon, and a few other 

 metals, sometimes disseminated in, at other times imbedded, are 

 the constituents of compound rocks, which often present an uni- 



