644 GEOLOGY. 



nature. Hence we have the Granitic, Trappean, and Volcanic rocks the popular 

 distribution from which there is no necessity here to deviate. 



GRANITIC ROCKS. Pliny uses the word geranites to denote a particular kind of 

 stone, to which some writers refer the word granite, but more probably the granular 

 structure of the rock itself originated the term. Its essential ingredients are quartz, 

 felspar, and mica ; the felspar in general predominating, and the mica occurring in the 

 smallest proportion. These constituents vary considerably in size, being sometimes very 

 coarse, and in other cases so fine as to be scarcely visible to the naked eye ; and between 

 these extremes there exists an almost infinite variety. The fine-grained varieties are the 

 best adapted for economical purposes ; but the coarser abound most in interesting simple 

 minerals. The ingredients of granite vary also as to colour, and that in the same in- 

 gredient. Felspar occurs usually dark-red or white ; quartz, white or grey ; and mica, 

 black, brown, or white, and in various degrees to silvery. Hence the different hue exhi- 

 bited by the rock, from the flesh-coloured granite of Scotland to the white of Cornwall. 

 The coping stones of Waterloo Bridge show the red, and the balustrades the white variety. 

 The preceding ingredients enter into the composition of all true granite, but frequently 

 one is unusually predominant, or entirely wanting and without a representative ; or talc, 

 hornblende, or hypersthene supply its place ; or one of these is added to the three con- 

 stituents, and hence the following varieties : 



Granite of Two Ingredients, 



Felspar and mica ; quartz and mica, either uniformly mixed, as in Muncaster Fell, 

 Cumberland, or in segregated portions, constituting graphic granite ; quartz and horn- 

 blende ; felspar and hornblende, common in Aberdeenshire. 



Granite of Three Ingredients. 



Quartz, felspar, and mica, uniformly blended, constituting the true specimen, or with 

 distinct additional crystals of felspar, composing porphyritic granite, a fine example of 

 which occurs near Shap in Westmoreland; quartz, felspar, and mica, the quartz and 

 mica very rare, and the felspar predominant, forming felspathic granite, the whitestone 

 of Werner, eurite of the French, and compact /elspar of English geologists ; quartz, 

 felspar, and talc or chlorite, the composition of the granite of Mont Blanc; quartz, 

 felspar, and hornblende, the Sienitic granite ; mica, quartz, and hypersthene. 



Granite of Four Ingredients. 



Quartz, felspar, mica, and hornblende, or actinolite; quartz, felspar, mica, and compact 



felspar, or porcelain clay ; quartz, felspar, horn- 

 blende, and chlorite, or steatite. 



Graphic granite, a binary composition, is so 

 called from an arrangement of the quartz and 

 the felspar, which gives to the surface of the 

 rock, upon being polished, the appearance of an 

 inscription with Arabic or Oriental characters. 

 The accompanying cut represents an example from 

 the northern United States. It is only found in 

 veins. One of its best-known localities in our 

 own country is Portsoy, on the coast of Banff- 

 shire, where a vein occurs in mica-slate. 

 Sienitic granite, a ternary compound, in which hornblende, either wholly or partially, 

 supplies the place of mica, giving a darker hue to the mass, has received its name from 

 Syene, in Upper Egypt, where it is found in great abundance, and is the common mate- 

 rial of the ancient monuments in that country, the Pyramids and Pompey's Pillar. When 

 it was ascertained that Mount Sinai, in Arabia, was composed of it, one of the French 



