On Mineral Metamorphi8m% 3G0 



role of watery vapours, impregnated with sulphur, particu- 

 lai'ly attracts the attention, by the strikingly altered colour- 

 ing of the soil: the face of a bold projecting felspathic 

 lava is changed into a white, coarse-grained, chalky marl 

 or tripoli-like rock ; the adjacent tuffa, which is yellowish- 

 white, very friable, and frequently intersected by dark-red 

 streaks, incloses coarse lumps of a bluish-white stone, re- 

 sembling opal or pitch-stone, the numerous crevices of which 

 are lined Avith chalcedony or hyalite. 



[\Ve may add, as in some degree connected with the preceding, that the for- 

 mation of many minerals by sublimation in smelting processes, and in active 

 volcanoes and solfateros, is well known. Thus the rents in the walls of smelt- 

 ing furnaces are frequently filled or encrusted with metallic substances which 

 could only reach these rents by sublimation. Crystals of lead-glance, and zinc- 

 blende occur, on the walls and in the rents of furnaces where lead and 

 zinc ores are smelted; and beautiful cubes of titanium are found in the walls 

 of furnaces where titaniferous iron-ore is smelted. Graphite occurs in the ve- 

 sicular cavities, or on the surface of iron slags, and appears in such to be a 

 sublimation of carbonaceous matter. In nature we find graphite associated 

 with minerals which many consider as formed at high temperatures, such as 

 felspar, quartz, and mica ; also in metamorphic marble and trap. Thus the fa- 

 mous graphite of Borrodale in Cumberland occurs in irregular nests in vertical 

 veins in trap. The vein stones are calc-spar, brown-spar, and quartz.] 



Metamorphosis of Rocks by Injection. 



As on a grand scale, veins of fiery molten substances have 

 filled up the interstices of sedimentary rocks, so in several 

 cases narrow veins of molten substances appear to have 

 penetrated into the rocky mass in such quantities, and so 

 universally, that the rock acquires a wholly altered charac- 

 ter, since the newly injected substance established itself as 

 an essential ingredient of the rock. In the case of many 

 quartzites in the flysch regions of "Wallis and Grandbiindten, 

 this origin becomes not at all unlikely, when we see the veins 

 of quai'tz, in the flysch. becoming more and more numerous, 

 until, at last, only a few remaining thin slaty plates of flysch 

 indicate the nature of the original rock. Quartz nodules also, 

 as we frequently find them in clay-slate and mica-slate, may 

 have arisen if the injected fluid was made to separate in 

 consequence of the resistance of the mass penetrated. Masses 

 of gneiss, which are penetrated by veins of granite or of por- 

 phry, have often the appearance of having been formed by 

 the forcible entrance of felspath and quartz between the 

 layers of the original slate. Through the influence of the 



VOL. XLIV. NO. liXXXVIII. — AlMUIi 1848. 2 A 



