352 



MINERALS AND GEOLOGY 



merly assigned to them, both at this spot and at localities along the 

 Bay of Chaleurs, are now regarded as Devonian * 



Eruptive Rocks : The more important eruptive rocks occuring 

 within the Appalachian and Gaspe' District, comprise : (1) the tra- 

 chytic mountains of Brome and Shefford ; (2), the granites of the 

 Great and Little Megantic Mountains, and other granitic masses of 

 the Eastern Townships; and (3),, the trappean dvkes and hills of 

 Eastern Gasp4 and the Bay of Chaleurs. 



The Brome and Shefford mountains consist essentially of granitoid 

 trachytes, composed of greyish-white or light-coloured feldspar with 

 scales or grains of brown or black mica and dark hornblende, and 

 some minute crystals and specks of sphene and magnetite. The feld- 

 spar, according to Dr. Sterry Hunt's analyses, contains both potash 

 and soda but shews the cleavage of orthoclase. The Brome (and 

 Gale) Mountain covers an area of about twenty square miles, and the 

 closely adjacent Shefford Mountain, about nine square miles. Both 

 belong to the same eruptive series as the trachytic and trappean 

 mountains (Yamaska, Chambly, Montreal, etc.) of the Upper St. 

 Lawrence District described on a preceding page, all of which are 

 apparently of Upper Silurian or Devonian age. 



The granites of this region are also of Devonian or later age. They 

 present a very general resemblance in aspect and composition, consist- 

 ing almost uniformly of white orthoclase-feldspar and colourless 

 quartz, with small quantities of black or brown mica. Large ex- 

 posures occur in Stanstead, one alone covering five or six square 

 miles ; also in the country immediately south-west of Lake St. Francis, 

 and around Lake Megantic in Compton ; and smaller exposures are 

 seen in Barnston, Burford, Hereford, and other townships. In most 

 cases the granite has penetrated the surrounding rocks in veins or 

 dykes, and has greatly altered these : rendering the limestones and 

 dolomites more or less crystalline, with partial or complete oblite- 

 ration of iossils ; and converting the slates in some places into mica- 

 ceous schists, or developing in them crystals of chiastolite (see page 

 97), garnet, hornblende, and small cubes of pyrites, as well as pro- 

 ducing fractures and other signs of mechanical disturbance. 



The eruptive dykes of Gaspe consist partly of grey dolerites (in 

 places amygdaloidal), and in part of green diorite with porphyritic 



* For structural and other details, see Report on the Gaspe Peninsula by R. W. Ells, 1882 



