THE CRUST OF THE EARTH. 



oxides of iron and manganese are occasionally mixed with this 

 constituent of granite, and though present in extremely minute 

 quantity, produce nevertheless a very striking appearance, render- 

 ing the rock white, cream-coloured, or red, according to their 

 varying proportions. 



MICA is the shining glossy particles of the stone, which reflect 

 light like bits of glass or metal. The name is derived from 

 "MICANS," glittering. Mica may be seen in many other stones, 

 as also in sand. 



QUARTZ, which appears in granite in the form of white crystals, 

 is the substance known as SILISX or SILICA, or the EARTH OF 

 FLINTS, and is one of the hardest and most abundant of mineral 

 substances, entering largely into the composition of many other 

 mineral masses. It is from this constituent, chiefly, that granite 

 receives its hardness. 



Silica is familiarly known as ROCK CRYSTAL. 



22. It must be remembered that the several materials of which 

 the igneous rocks are thus composed, are not combined together 

 chemically. They are not combined for example in the same 

 manner as are sulphur and oxygen, when these constituents pro- 

 duce vitriol. They are on the contrary merely agglomerated and 

 brought into mechanical juxtaposition, forming a solid mass by 

 the mere cohesion of crystal to crystal, so that by the action of 

 mechanical force it would be possible to resolve the rock into its 

 component parts. 



23. Each of these constituents is,' however, itself a compound. 

 Thus feldspar is a compound of the silicates of several chemical 

 substances, such as alumina, lime, and potash, or soda that is, it 

 is a combination of these severally with silicic acid. 



Mica is composed of like silicates, with the addition of silicate 

 of iron. 



Quartz is in fact silicic acid itself. It will appear, then, from 

 this statement, how important a part silex, or earth of flints, 

 plays in the formation of the globe. 



24. If the constituents of the igneous rocks were combined, 

 one with another, chemically, instead of being mechanically 

 juxta-posed, they would, according to a general law of nature, 

 always be found to prevail in the same rocks in one invariable 

 numerical proportion; but being, as explained above, merely 

 agglomerated by cohesion, without any chemical union, they may 

 exist in any proportion whatever, and hence have arisen a cor- 

 responding variety of granites. In some specimens the quartz 

 and mica are altogether absent, and then the granite, consisting 

 of feldspar only, in the pasty and crystallised state, takes the 

 name of PORPHYRY. 



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