428 INTRUSIVE ROCKS OF THE DISTRICT OF MONTREAL. 
show them to consist of orthoclase mingled with more basic feldspars, 
or with hydrated silicates like natrolite, thus passing into phonolites. 
The accidents of structure which are supposed to characterize this 
class of rocks are however so little dependent upon chemical compo- 
sition that in many of the so-called trachytic rocks of Hungary and 
Guadaloupe the predomimant mineral is a basic feldspar like labra- 
dorite, containing large amounts of lime and soda, with but little 
potash. 
Among the trachytic rocks of Lower Canada, I have met with none 
which are porous or vitreous The white trachytic dykes at Lachine 
are finely granular, and sometimes earthy in texture ; they occasionally 
assume a concretionary structure, and are often porphyritic from the 
presence of crystals of feldspar. The reddish-gray trachytic porphyry 
of Chambly offers an example of well-defined feldspar crystals in a 
paste consisting of finely lamellar orthoclase with a slight excess of 
silica and small portions of mica. Several dykes about Montreal 
consist of a trachytic porphyry with large feldspar crystals in a com- 
pact purplish or lavender-gray base of a waxy lustre, which effervesces 
with acids from an admixture of carbonates, and closely resembles in 
appearance certain trachytes from the Siebengebirge upon the Rhine. 
Other varieties can hardly be distinguished from the so-called domite, 
the trachyte of the Puy de Dome, and exhibit small drusy cavities. 
The presence of carbonates in trachytes has generally been overlooked ; 
Deville, however, found seven per cent. of carbonate of lime in a tra- 
chytic rock from Hungary, and I have observed it disseminated in 
some of the trachytes of the Siebengebirge. 
In my report already referred to, I have shown that some of the 
trachytes of our vicinity apparently contain carbonates of magnesia 
and iron, and perhaps of manganese, in addition to carbonate of lime. 
Many of these rocks weather to some depth of a reddish-brown from 
the peroxydation of the iron. One of this kind, which forms a large 
dyke in the limestones at the Mile-End Quarries, is remarkable for its 
large proportio of carbonates. It is grayish-white with dark gray 
spots, granular, sub-vitreous in lustre, and has the aspect of an im- 
pure quartzite. It loses by ignition 11-0 per cent. of its weight ; 
reduced to powder it effervesces freely with nitric acid, disengaging 
carbonic acid, which when heat is applied is mingled with nitrous 
fumes from the peroxydation of the iron. 100 parts of the rock gave 
in this way to the acid 4°84 of alumina, besides lime, magnesia and. 
iron, which represented as carbonates equalled carbonate of lime 11°60, 
