206 _ GEOGNOSY AND GEOLOGY. 
Among accessory phenomena may be cited the occurrence of breccias, 
of oxyde of iron, of silicious rocks, as hornstone, whet and silicious slate, 
which often, injected into limestone, give to it the appearance of marble ; 
also, the occurrence of gypsum in the vicinity of diabase. 
Pyroxene rocks are of very general distribution; they are found in 
Sweden, Norway, Great Britain, Germany, in the Pyrenees, France, south 
coast of Spain, in the Apennines, and in North America. 
Among these masses are also to be enumerated schiller spar, ophite, and 
serpentine. Ophite, in particular, occurs where serpentine stands in 
connexion with marble or dolomite. Serpentine is extensively distributed 
in Turkey, where it has many external features in common with 
euphotide. 
Serpentine is exceedingly interesting, from the mineral substances which 
it incloses. Among these are chromate of iron, used in the preparation of 
the pigments of chrome : also, platinum, pyrites, masses of asbestos, broncite, 
picrolite, chalcedony, opal, &c. 
The weathering of serpentine proceeds very slowly, but is much facilitated 
by the dissemination of iron pyrites. The resulting sulphuric acid combines 
with a portion of the magnesia contained in the serpentine, and forms 
sulphate of magnesia or epsom salts. 
2. Voucanor Rocks. 
The volcanoid masses coincide more with the volcanic than with the 
plutonic : they are intermediate between the two, and traverse the latter, 
but are never pierced by them. The pyrotypic character is very distinctly 
impressed on them, and while pure silex diminishes in quantity, oxyde of 
iron occurs in so much greater proportion. The principal rock species are 
trachyte, phonolite (clinkstone), and basalt. 
A. Trachyte. 
This is often accompanied by subordinate masses of pearl, pitch, and 
pumice-stone and obsidian, as also by hornstone and claystone-porphyry, 
and possesses very striking mountain forms, of a bell shape, or that of a cone 
either acute or truncated. Trachytic rocks lie either in linear series one 
after another, or they are grouped concentrically. They sometimes attain 
a considerable height, as the Mont d’Or in Auvergne. _ Its absolute height 
is 3000 feet, and the entire. elevation above the level of the sea 5800 feet. 
The cleavage is sometimes conformable to the mountain outline; often, 
however, columnar or bench formed, as on the Wolkenburg in the 
Siebengebirge (pil. 49, fig. 6). 
The veins of trachyte are often of importance ; some of them contain gold 
and silver. = 
Trachyte masses emerge in the most different normal formations; they 
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