170 THE CANADIAN NATURALIST. [June, 



seems to be made up of lamellae of ortlioclase, mingled with small 

 portions of carbonates of lime and magnesia. A part of the iron 

 also is probably present as carbonate, which, by its decomposition, 

 gives rise to the rusty red color of the weathered surface of the 

 trachyte. 



Montreal. — The island of Montreal offers a great variety of 

 trachytic rocks, which traverse both the Lower Silurian strata, and 

 the dolerite of Mount Royal. Some of these dykes are finely 

 granular, occasionally crumbling to sand, and frequently are earthy 

 in texture. In some cases they assume a concretionary structure, 

 and they are often porphyritic from the presence of feldspar or 

 hornblende. One variety exhibits large feldspar crystals in a com- 

 pact purplish or lavender-gray base, with a waxy lustre. This 

 effervesces with acids, from an admixture of earthy carbonates, and 

 closely resembles in its aspect certain trachytes from the Siebenge- 

 iDirge on the Rhine. Other varieties can scarcely be distinguished 

 from the so-called domite, the trachyte of the Puy de Dome, and 

 exhibit small drusy cavities. The presence of carbonates in tra- 

 chytic rocks has generally been overlooked ; Deville however found 

 seven per cent of carbonate of lime in a trachytic rock from Hun- 

 gary, and it occurs disseminated in some of the trachytes of the 

 Siebengebirge. Some of the trachytes about to be described con- 

 tain moreover carbonates of magnesia and protoxyd of iron, and 

 weather to some depth of a reddish-brown color from the peroxy- 

 dation of the latter, like the trachyte from Chambly just noticed. 

 Acids remove from many of these rocks, in addition to the carbo- 

 nates, portions of alumina and alkalies. These are derived from 

 a soluble silicate, which in the trachytes of Brome appears only 

 as rare crystals of nepheline, and in Chambly as analcime and 

 •chabazite. In some of the compact and earthy varieties about 

 Montreal, however, this soluble silicate exists to a large extent, 

 and has the composition of natrolite. By this admixture of a 

 zeolite the trachytes pass into phonolite. 



The first of these trachytes which will be noticed forms a dyke 

 near McGrill College. The rock is divided by joints into irregular 

 fragments, whose surfaces are often coated with thin-bladed crys- 

 tals of an aluminous mineral, apparently zeolitic. Small brilliant 

 crystals of cubic iron-pyrites, often highly modified, are dissemi- 

 nated through the mass. The rock has the hardness of feldspar, 

 and a specific gravity of from 2.617 to 2,632. Its color is white. 



