190 



MINERALS AND GEOLOGY 



with active or extinct volcanic cones, are actual lavas in the cor 

 sense of the term ; but others, although undoubtedly of similar 

 origin, occur in localities to which the term volcanic lias ceased to 

 apply. Viewed generally, although no marked lines of demarcation 

 can be drawn between them, the Trachytes present the following 

 leading varieties : Common or Porous Trachyte ; Compact or 

 Massive Trachyte ; Slaty Trachyte ; Granitoid Trachyte. Examples 

 of porphyritic structure occur in each of these varieties ; and in the 

 trachytes of some localities the feldspar consists partially or wholly 

 of soda or lime species. 



Common Trachyte is met with chiefly in regions in which active 

 or extinct volcanoes are distributed. It is more or less porous, or of 

 an open granular texture, and is frequently porphyritic from enclosed 

 crystals of glassy feldspar. Scales and specks of mica, (fee., are some- 

 times scattered through it, and it contains occasionally some grains 

 of quartz. The latter mineral is altogether of exceptional occurrence, 

 but its occasional presence serves to connect the trachytes with granitic 

 rocks. Compact Trachyte, also known as "white trap" or "feldspar 

 trap," occurs in broad veins or dykes traversing both the older trap 

 or dolerite of the Montreal Mountain and the Lower Silurian lime- 

 stones of that neighbourhood. It contains at these localities a con- 

 siderable amount of intermixed carbonate of lime ; whilst a related 

 variety of somewhat slaty structure, from near Lachine, is partly 

 zeolitic in its composition, and would thus be known by many lith- 

 ologists as a phonolite. These examples are partially in an earthy 

 state, a condition sometimes recognized by a special name, that of 

 Domite, a term applied to the earthy or semi-decomposed trachytes 

 of the Puy-de-D6me in the ancient volcanic district of Central France. 

 A porphyritic variety of pale-red or yellowish trachyte, holding large 

 crystals of feldspar, occurs also at Chambly. Examples of Granitoid 

 Trachyte are especially abundant in the Eastern Townships of Brome 

 and Shefford where they form eruptive passes of considerable extent 

 and elevation. The trachytes of these mountains are both eoarse 

 and fine granular, and are composed of orthoclas j or other feldspars 

 with intermixed scales of black or brown mica, grains of yellow 

 sphene and magnetic iron ore. Some crystalline particles of black 

 hornblende are also occasionally present. In the Yamaska Moun- 

 tain of the same district, a micaceous rock of this character changes 



