1864.] T. STERRY HUNT ON LITHOLOGY. 181 



one p irt, while in another the augite and olivine predominate. By 

 the action of the weather, the feldspar acquires an opaque white 

 surf ice, upon which the black shining; au-i:ite and the rusty-red 

 decniiiposinor olivine appear in strong contrast. 



The dolerite of this mountain is traversed by numerous dykes, 

 some of which are diorites like those of Monnoir and Beloeil, about 

 to be described. A tlvke of compact dolerite holding crystals of 

 feldspar and grains of olivine, is found intersecting the strata of the 

 Hudson River formstion at St. Hyacinthe. 



M HJMT Royal. — This hill which rises immediately in the rear 

 of Montreal, consists for the most part of a mass of highly augitic 

 dolerite. In some pirts large crystals of augite, like those of 

 Mont irville, are disseminated through a fine-gr •in'^d base, which 

 is dirk ash-gray in cilor, and often efFerveFC'?s freely with acids, 

 from the presence of a portion of intermingled cirbonate of lime. 

 At other times this is wanting, and the rock is a mass of black 

 crystalline augite, constituting a veritable pyroxenite, from which 

 feldspar is absent. Mixtures of augite with feldspar are also met 

 with, constituting a granitoid dolerite, in parts of which the feld- 

 spar predominates, giving rise to a light grayish rock. Portions of 

 this are someti es found limited on either side by bands of nearly 

 pure black pyroxenite, giving at first sight an aspect of stratifica- 

 tion. The bands of these two varieties are found curiously contorted 

 and iiiterupted, and as at Montarville, seem to have resulted from 

 movements in a heterogeneous pasty mass, which have efiected a 

 partial blending of an augiticmagma with another more feldspathie 

 in its nature. 



The more augitic parts of Mount Royal contain, like the similar 

 varieties from Rougemont and Montarville, considerable portion! 

 of magnetite, and some ilmenite. At the east end of the mountain 

 a variety of dolerite, containing olivine, occurs. It consists of a 

 base of grayish-white granular feldspar, which m the specimen ex- 

 amined constitutes about one half of the mass, and encloses crystals 

 of brilliant black augite, and of semi-transparent amber-yellow oli- 

 vine. Tliis rock closely resembles the feldspathie peridotite of 

 Rougemont, described above ; but the imbedded crystals are some- 

 what larger, although less than those in the dolerite of Montarville, 

 A portion of the feldspar, freed as much as possible from augite, 

 furnished by analysis the result already given under XIX; which. 

 shovvs that it approaches labradorite in composition. 



