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 ‘ Domite.”’ 
This latter term is derived from the partially-decomposed trachytes of 
the Puy-de-Déme, in central France. 
4. Traps and Greenstones.—The rocks of this group chiefly affect 
the form of intrusive dykes (2.e. 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 researches 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 J'rap. 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 
erystals 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, here and there, on the north shore 
of Lake Superior, and probably in other parts of the Province. 
When, again, as frequently happens, a trap or basalt is of a more or 
* The student should refer to the descriptions of these minerals in the preceding Part of 
this Essay. See the Index, pages 162-4, of this volume. 
