MINERALOGY. 



413 



CHAP. V. 



We have already, in Chap. IV. established five classes 

 of rocks, distinguished by the names primitive, tranti- 

 tion, tecondary, alluvial, and volcanic. We shall now 

 describe these in the order just mentioned, begin- 

 ning therefore with those of the primitive class. 



CLASS I. PRIMITIVE ROCKS. 



Urgebirge. Werner. 



Terrains Primitifs. Daubiiiston. 



Primitive rocks are those whose period of formation 

 is considered as antecedent to that of the creation of 

 organic beings. Hence we arrange in this class all 

 those formations which have not been found to contain 

 petrifactions or fossil organic remains. Should fu- 

 ture observations, however, prove that these rocks do 

 occasionally inclose animal or vegetable remains, still it 

 would be well to consider them as a distinct class, and 

 retain the name primitive, because from their lying un- 

 der the rocks of the other classes, they are to be consi- 

 dered as having been fanned before them, and may, 

 therefore, be said to be primitive, or firt formed. 



They are of chemical formation ; and if mechanical 

 deposites do occur associated with them, the quantity is 

 inconsiderable. All those kinds which abound in mica 

 are distinctly stratified, while the others that do not 

 contain it are less distinctly stratified. The direc- 

 tion of the strata is frequently parallel with that of the 

 mountain range in which they are contanmrt. 



All the different rocks of the class alternate with, and 

 pass into each other. 



The constituent parts are quartz, felspar, mica, lime, 

 stone, and hornblende, minerals indeed of which near- 

 ly the whole mass of the upper coat of the earth is com- 

 posed. Of these minerals, the felspar, quartz, mica, 

 and hornblende occur together, and in various forms of 

 aggregation, while the limestone forms beds of greater 

 ocless extent, which are but slightly intermixed or con- 

 nected with the boundary strata. 



The following are the rocks of this class : 



1. Granite, with sienite, protogine, and topaz-rock. 



S. Gneiss, with some varieties of white stone. 



3. Mica-slate, with different varieties of talc-slate. 



4. Clay-slate, with alum-slate, flinty slate, Sec. 



5. Granular limestone, and primitive gypsum. 



6. Primitive trap. 



7. Serpentine and euphotide. 



8. Porphyry. 



9. Quartz-rock. 



I. Granite. 

 Granite. Werner. 



Saxum, quartzo spato scintillante et mica, in di versa 

 . proportione mixtis compositum. Granites, ll'alleriiu. 



Granite of the Germans, French, &c. Moon tone of 

 Cornwall. Whinstone and sandstone in some part* of 

 Scotland. 



Nm. 1 The name granite is a corruption of the Latin word 

 geranila, used by Pliny to designate a particular spe- 

 cies of stone. The fim modem writer who uses this 

 word is Tourneforte, the celebrated naturalist. It oc- 

 curs in the Account of fui Voyage to the Levant, pub- 

 liahed m \hn'j. Andqwmans' appear to have named 

 'ery granular itone capable of being used in architec- 

 ture or statuary, granite, and it continued to be used in 



this vague sense by mineralogists until about fifty years Geognoty, 

 ago, when true granite was distinguished as a particu- ""-V 1 

 lar mountain-rock. 



2. It is a rock composed of grains or concretions of Constiiu- 

 felspar, quartz, and mica intimately joined together, nt parti, 

 but without any basis or ground. These parts vary in 

 quantity, so that sometimes one, sometimes the other, 

 and frequently two of them, predominate. Felspar is 

 generally the predominating, as mica is the least consi- 

 derable, ingredient of the rock. In some varieties, the 

 quartz is wanting, in others the mica, and these have 

 received particular names. Such distinctions, however, 

 are useless, as these masses are to be considered as mere 

 varieties, not distinct species. 



The constituent parts differ also considerably in their 

 magnitude : they alternate from large to small, and 

 even very fine granular. In some varieties, the concre- 

 tions of felspar and quartz are several inches in size, 

 and the mica is in plates upwards of a foot square ; while 

 in others, the grains are so small that the granite ap- 

 pears nearly compact. 



It differs also considerably as to colour ; and this de- 

 pends principally on the predominating ingredient, the 

 felspar, the quartz and mica having usually a grey co- 

 lour. The felspar is usually white, and most common- 

 ly greyish and yellowish white ; also reddish or milk- 

 white; sometimes also flesh-red. It is seldom grey, 

 yellow, or green. The quartz is usually grey, seldom 

 milk-white, and always translucent The mica is usu- 

 ally grey, and sometimes nearly black. 



The felspar in granite has usually a vitreous lustre, 

 and perfect foliated fracture, yet in some varieties it 

 passes into earthy, with the loss of its lustre and hard- 

 ness; in short, it has passed into porcelain earth. This 

 appearance is sometimes produced by the weathering 

 of the felspar ; sometimes it appears to be its original 

 state. When veins containing pyrites traverse granite, 

 the felspar and mica in their vicinity are converted into 

 a kind ofsteatitical matter, by the action of the sulphu- 

 ric acid formed during the decomposition of the pyrites. 

 The mica sometimes also decomposes by exposure to 

 the atmosphere ; but the quartz is never altered. Gran- 

 ite, with earthy felspar, is found in Cornwall. 



Sometimes the constituent parts of granite are regu- Graphic 

 larly crystallized ; principally, however, the felspar andg r *nite. 

 quartz. The mica sometimes occurs in nests, unmixed 

 with the other constituent parts, and these have been 

 confounded with fragments. Sometimes the constitu- 

 ent parts are so arranged, that when the specimen is 

 cut, its surface has a kind of resemblance to written cha- 

 racters : hence this variety has been denominated graphic 

 itone. This particular variety is found at Portsoy, and in 

 the island of Arran ; also in the forest of Thuringia in 

 Germany, in Corsica, in the mountain of Odontscbe- 

 long, in the Uralian range, and also in France. Granite, 

 with regular crystallized felspar, occurs in the island of Radiated 

 Arran, in many places on the continent of Europe, andr*" 1 "- 

 also in South America. At Mount St. Gothard, all the 

 three constituent parts are found crystallized together. 

 In the island of Arran and the Saxon Erzgebfrge, there 

 is a remarkable variety of granite, in which the felspar, 

 quartz, and mica, have a diverging radiated structure. 



Several other varietinoi granite have been described 

 by g*ognosts. Of these, the two most important are 

 protogine and itenilc. We shall now describe them. 



ProUigine. 



In this variety, talc, either in the lamellar eom-Protoflne. 

 pact, or steatitic form, or as chlorite, takes the place 



