ON THE FOEMATION OF CRYSTALLINE ROCKS. 2G3 



Among the minerals which are most frequently formed in argilla- 

 ceous schists are made, or chiostalite, stauroticle, distheno, mica 

 belonging to two species, which is often in very small spangles ; the 

 feldspars, orthose, and anorthite ; amphibole, which is sometimes 

 sufficiently abundant to constitute an amphibolic schist,* tourmaline, t 

 <fec. These minerals are generally found in the neighborhood of 

 granite. It is chiefly in limestone that a great variety of minerals 

 have been found, among which I will mention garnet, idocrase, am- 

 phibole, wollastonite, epidote, paranthine, dipyre, couzeranite, mag- 

 nesianmica, gehlenite, chondrodite, spinel, | serpentine, talc, chlorite, 

 seladonite, the zeolites, certain clays, &c. These different minerals 

 do not, however, belong exclusively to calcareous rocks. § Thus the 

 zeolites are found not only in limestones, but also in argillaceous 

 rocks, sandstones, and sometimes even in mineral combustibles, when 

 these rocks have been traversed by eruptions of trap.ll In the 

 neighborhood of all kinds of eruptive rocks, granite and others, 

 quartz is often accumulated, either in crystalline or compact masses, 

 or as jasper.^ This ubiquity belongs, also, to other minerals of 

 metallic veins, such as the carbonates with lime, magnesia, and iron, 

 as bases, sulphate of baryta, fluor spar, and specular iron.** As ex- 

 amples of this action, the varieties of which are numberless, I men- 

 tion the classic locality of the Hartz, where the schist that borders 

 on the granite (hornfelsj contains mica, feldspar, tourmaline, chlorite, 

 garnet ;tt Cornwall, where effects of the same kind are produced ::{:| 

 the Vosges,§§ the Pyrenees, Britanny, !lil Norway, &c. 



Sometimes the rocks bordering on granite or syenite are modified 

 to such an extent that they themselves assume the character of 

 eruptive rocks. Thus, in the Vosges, argillaceous schist passes by in- 

 sensible gradations to a feldspathic rock, which is sometimes porphyro- 

 idal, and to green porphyries studded wnth anorthite and amphibole. 

 Similar facts have been observed in other countries.^^ The trans- 

 formed rock often becomes amygdaloidal ; in certain parts of Ger- 



■™" Environs of Christiania. 



•)• Hornfels iu the Hartz. 



j Monzoni, Somnia ; the Silurian limestone of Sparta, in the United States. 



§ Some, however, as wollastonite and gehlenite, have as yet only been found in lime- 

 stone. 



II Tertiary limestone of the conglomerate of Puy de la Piquette, marls of the Cyclops 

 islands, containing beautiful crystals of analcime, argillaceous schists of Andreasberg, in the 

 Hartz, and of the Isle of Anglesey, tertiary sandstoue of a vitrified appearance in Wilden- 

 stein, in Veteravia. Zeolites have even been formed iu granite, near veins of basalt which 

 traverse it, in the Isle of Arran, for instance. — (Bou6 — Essai Geologique siir I'Ecosse, p. 499 ; 

 and at Haustein, in the Forot-Noire ; (Schill, JVeues Jarbuch, 1857, p. 36.) 



*[[ Tuscany, Greece, Ural, &c. 



•~'^" Sometimes quartz has been simply isolated by the decomposition of pre-existiug sili- 

 cates, as we shall sec m the third part ; sometimes it results, as in the other gangues of 

 metalliferous veins, from a manifest deposit. 



ft According. to Hoffman and Zincken. 



%X Oe la Beche. — {Geological Report on Cormoall, p. 267.) 



§§ Daubree'. — (DefcripH^n Geologique du Bas Rliiii, pp. 32 and 52.) 



nil According to the pre-cited memoirs of Palassou, Dufrenoy, and of Durocher.^ 



^^ Bulletin de la SociM Geologique, 1st series, vol. vii ; Comptes Rendus, vol. xix, p. 857; 

 Reisnach Our^l, vol. ii, p. 185. 



