510 J. A. DRESSER IGNEOUS ROCKS OF EASTERN QUEBEC 



series of irregular hills which are instrusive through most of the other 

 rocks of the region, generally Paleozoic sediments. They occur notably 

 in the counties of Brome, Sherbrooke, Kichmond, Wolfe, Megantie, and 

 probably in Dorchester, and are likewise known to reappear in the Gaspe 

 highlands. Among them are the Bolton mountains, Owls head, Orford, 

 Ham, and Adstock mountains. The rocks composing them are serpen- 

 tines, diabase, gabbro-diorites, with frequent or smaller masses of horn- 

 blende granite and occasionally much smaller bodies of porphyrite. Of 

 these rocks the serpentine seems to be in all cases the oldest, being cut 

 by intrusions of the other, and probably it is older than the paleozoic 

 sediments. The diabases and gabbro-diorites, which are phases of the 

 same magma, form the greater part of all these hills. They are later in 

 age than the serpentines through which they are commonly intrusive. 

 The hornblende granite in some instances is distinctly intruded through 

 the serpentines, but in one case at least, the Big Ham mountain, it seems 

 unmistakably to have been differentiated from the parent magma of that 

 rock in situ. 



The porphyrite is of limited occurrence, but at Shipton pinnacle, where 

 it is best seen, it seems to cut the serpentine. It generally occurs as the 

 matrix of a band of breccia seldom exceeding 300 yards in width that is 

 frequently found along the southern edge of the serpentine belt. As far 

 as it has been studied, it is found to be a quartzless porphyrite containing 

 a little hornblende as the only ferromagnesian constituent. 



Mount Orford (2,860 feet) is the best known as well as the largest of 

 the gabbro-diorite and diabase hills. It has an area of not less than 20 

 square miles and an average height above the surrounding country of 

 1,000 feet. It comprises two main divisions which give the following 

 cross-section, measured westward along the line of the Canadian Pacific 

 railway near Miletta: Diabase or gabbro-diorite, 7,837 feet; graywacke, 

 165 feet; serpentine, 577 feet; sandstone, 82 feet; serpentine, 1,567 feet. 

 This section is bounded by sedimentary rocks on either side. The rock 

 of the first and greater mass is a uniform green color and shows gray 

 grains on a freshly broken surface. Quartz veins are common, and the 

 joint, plants and seams are often studded with small quartz crystals. 

 Patches of epidote sometimes as much as a foot in diameter are numerous. 

 The texture of the rock becomes finer toward the outer edge and also 

 toward the top of the mountain, where the cooling of the igneous mass 

 has taken place more rapidly. 



In thin-section this rock, which is exceedingly altered, shows plagio- 

 clase feldspar with aggregates of the pyroxenic decomposition products, 

 whose relation one to another indicates that the rock had the structure 



