. The Canadian journal of industry, science and art. MINERALS AND GEOLOGY OF CANADA. 429. carbonates. By weathering, also, they become reddish-brown, dull- white, &c., and tend to decompose into clay-stone or " ; This latter term is derived from the partially-decomposed trachytes of the Puy-de-D6me, in central France, 4. Traps and Greenstones.—The rocks of this group chiefly affect the form of intrusive dykes { broad and more or less straight or simply-forking veins (as in fig. 52), or otherwise occur in overlying, intercalated, and irregular masses, which frequently p
. The Canadian journal of industry, science and art. MINERALS AND GEOLOGY OF CANADA. 429. carbonates. By weathering, also, they become reddish-brown, dull- white, &c., and tend to decompose into clay-stone or " ; This latter term is derived from the partially-decomposed trachytes of the Puy-de-D6me, in central France, 4. Traps and Greenstones.—The rocks of this group chiefly affect the form of intrusive dykes { broad and more or less straight or simply-forking veins (as in fig. 52), or otherwise occur in overlying, intercalated, and irregular masses, which frequently present a colum- nar structure. The traps proper, or dolerites, are always of a black or dark colour, and consist essen- tially of a more or less uniform mixture of lime feldspar (or soda feldspar) and augite, with in gene- ral a mixture of zeolitic minerals and magnetic iron-ore. The green-stones or diorites, consist normally of soda feldspar (or of a feldspathic mixture) and hornblende, and have usually a more or less decided green colour. It is sometimes impossible, however, to distinguish greenstone from trap, more es- pecially as late researclu-s have shewn that augite and hornblende possess the same atomic composition. Hence the two rocks should properly be classed together. When the rock is of a black or dark colour, more or less compact, and amorphous in form, it is termed Trap. This variety occurs in numerous dykes on the north shore of Lake Huron and on the shores of Lake Superior. When a trap rock contains distinctly imbedded crystals of any mineral distributed through its mass, the name of this mineral may be conveniently attached to it. Thus, the Montreal mountain consists principally of Augitic Trap. The same variety, containing olivine* in addition, forms the mountains of Montarville and Rougemont. When the rock assumes a columnar or basaltiform structure, it becomes Basalt. This variety does not appear to be common in Canada, but it occurs, h^e and there, on th
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