184 THE CANADIAN NATURALIST. [June, 



is in white translucent vitreous cleavable grains; associated with 

 8m ill distinct prisms of black hornblende, scales of copper-colored 

 mica, and grains of magnetite. The analysis of the feldspar, 

 extracted by washing a portion of the crushed rock, and sti J con- 

 taining a little mica, is given above under XX iv. This result 

 approaches to thos obtained from the micaceous feldspar rock of 

 Yam: ska, y and VI ; which has been described as a kind of 

 trachyte, and with the rock of Belceil seems to constitute a passage 

 between the trachytes and diorites. 



R:gaud. — A portion of Rigaud Mountain consists of a rather 

 coars '-grained dior te, which is made up of a crystalline feldspar, 

 white or greenish in color, with small prisms of brilliant black 

 hornblende, and crystals of black mica. In some specimens the 

 feldspar, and in < thers the hornblende predominates. This rock 

 resembles the diorites of Beloeil and Monnoir. 



The granitoid dolerites of the Montreal grou , containing coarse- 

 ly ( rystall ne augite and olivine, break through the Lower Silu- 

 rian strata; and portions of these two minerals, probably derived 

 from these intrusive rocks, are found in the dolomitic conglomer- 

 ates near Montreal, which in some cases include masses of Upper 

 Silurian limestone, and are cut by dykes of a fine grained dolerite. 

 These, which perhaps correspond to the newer dykes of the same 

 rock at Grenville, show that there we.e at leas; three distinct 

 eruptions of dolerite, — one during the Silurian period, one before 

 it, and another after it. The trachytes of Montreal and Chambly 

 appear to be sti 1 more recent than these, and to traverse the 

 newest dolerites. 



The trachytes of Brome and Shefford seem to constitute a group 

 apart ; but the diorites of Yamaska Mud Mount Johnson, although 

 similar in aspect, differ widely in chemical composition. Facts are 

 gtill wanting to establish the geolo^zical age of these intrusive 

 masses. The different dolerites, which are related in mineral com- 

 position, belong as we have seen to different geological periods; 

 and i would not be safe to affirm that the different diorites or the 

 different trachytes of this vicinity are contemporaneous. Nor, on 

 the other hand, should even great discordances in chemical or 

 mineralogical constitution be necessarily regarded as establishing 

 a difference in the age of eruptive rocks. Evidence to the contrary 

 of this is Seen in the contiguous and intermingled masses of black 

 pyroxenite and grey feldspithic dolerite in Mount Royal and 



