844 THE WONDERS OF GEOLOGY. Lect. VIII. 



common mineralogical character, and with the exception of 

 the calcareous rocks, may have originated from the disin- 

 tegration and subsequent consolidation of more ancient 

 primary masses. 



There are various substances associated with this group, 

 as steatite, hornblende-schist, chlorite-schist which is a green 

 slaty mineral, and the beautiful mottled magnesian rock 

 called Serpentine; the latter is often connected with trap. 



Granite (so named from its granular structure), is the 

 foundation upon which all the strata of which we have 

 spoken are superimposed, and the framework of the earth's 

 crust; rising to the loftiest heights, and stretching into 

 mountain chains, which mark the grand natural divisions of 

 the physical geography of the globe. 



Although presenting great variety in the proportion and 

 colour of its ingredients, granite is essentially composed of 

 three substances, which may be easily recognised in the 

 blocks of which many of our pavements, bridges, roads, 

 and other works, are constructed. These are mica, known 

 by its silvery or glittering aspect ; quartz, by its glassy 

 appearance ; and felspar, which forms the opaque white, 

 pink, or yellowish masses, oftentimes seen in sections as 

 long angular crystals, which from their size and colour 

 may be readily detected, even by the unscientific observer. 

 In some species of granite, talc and hornblende occur, and the 

 mica is wanting ; these are called sienite, or syenite : those 

 masses which are composed of crystals of felspar, in a base 

 of earthy felspar, constitute porphyry, 



M. Bischof states that it may be demonstrated mathema- 

 tically, that all the sedimentary strata, and all the substances 

 enclosed in drusy cavities, are derived from the plutonic 

 crystalline rocks; these have furnished the materials, 

 and water has conveyed the quartz, calcareous spars, 

 heavy spars, metals, and other substances which fill such 

 cavities. 



