1864.] T. STERRY HUNT ON LITHOLOGY. 163 



rites of Rougemont and Mount Royal are cut by dykes of tra- 

 chyte. Similar dykes also traverse the diorite of Yamaska, and may 

 perhaps be connected with the trachytic portion of this mountain. 

 It is probable, judging from some specimens from Rougemont, that 

 the dolerite is there intersected by veins of diorite, some of which 

 resemble that of Beloeil, and others that of Monnoir. Dykes both 

 of trachyte, phonolite, and dolerite are also found traversing the 

 Lower Silurian strata in the vicinity of the great eruptive masses ; 

 and the conglomerate of St. Helen's mentioned above is traversed by 

 dykes of dolerite, which in their turn are cut by others of trachyte. 

 A second and smaller group of intrusive rocks occurs to the north- 

 west of Montreal, chiefly in the county of Grenville, where they 

 traverse the gneiss and limestones of the Laurentian system. The 

 principal undulations of these rocks have, like those of the Appa- 

 lachians, a north and south direction ; but there is apparent also 

 a second series of undulations, affecting in a less degree the geo- 

 graphical distribution of the strata, and having, like the Montreal 

 and Rigaud undulation, an east and west direction. Coincident with 

 the latter system of folds is a series of doleritic dykes, which nowhere 

 attain a great breath, but have in some cases been traced more than 

 fifty miles in a nearly east and west direction. These dykes are 

 interrupted by a great mass of reddish syenite, passing in some 

 parts into granite, and occupying an area of about thirty-six square 

 miles in the townships of Grenville, Chatham, and Wentworth* 

 Dykes of this syenite extend from the central mass, and traverse the 

 surrounding gneiss and limestone. Numerous dykes of quartz ifer- 

 ous porphyry intersect both this syenite and the surrounding gneiss, 

 and are seen in one case to proceed from a considerable nucleus of 

 porphyry, which rises into a small mountain ; rendering it probable 

 that numerous other porphyry dykes of the region radiate in 

 like manner from other nuclei of the same rock. Some parts of this 

 porphyry enclose fragments of syenite, dolerite, and gneiss, which 

 vary in size from small grains to several feet in diameter, and often 

 give to the rock the character of a breccia. In one instance a bed 

 of gneiss, upwards of a hundred yards in length, is completely sur- 

 rounded by the porphyry. 



Orthophyre and Syenite. 



Orthoclase-Porphyry or Orthophyre. — Under this head 

 may be noticed a rock which has for its base a compact petrosilex, 



