178 THE CANADIAN NATURALIST. [JlIDe, 



tarville, Rougemont, and Mount RoyaL In all of tliese however 

 great diversities of composition are met, with^ which will be suc- 

 cessively noticed, 



MoNTARViLLE. — The greater part of IMontarville is composed of 

 a coarse-grained granitoid dolerite, in which blac'-. cleavable nugite 

 predominates, — sometimes almost to the exclusion of any other 

 mi eral. Small portions of white feldspar, and scales of brown 

 mica, are sparsely scattered through the roek, with grains of 

 carbonate of lime. The removal of these by solution from the 

 weathered surface often gives to it a pitted aspect. In other portions^ 

 the feldspathic element predominates, and the rock becomes por- 

 phyritic from the presence of large crystals of augite. The worn 

 surfaces of the dolerite sometimes show alternations of this variety 

 with another which is finer-grained and whiter. The two are 

 arranged in banils, whose varying thickness and curving line^ 

 suggest the notion that they have been produced by the flow and 

 the partial commingling of two semi-fluid masses. 



Another and remarkable variety of dolerite, found at Montar- 

 ville, appears to be confined to a hill on the shore of the little lake 

 about half a mile northward from the manor-house. The whole 

 of this hill, with the exception of some adherent portions of indur- 

 ated shale, seems to be composed of a granitoid dolerite, containing 

 a large proportion of olivine. This mineral occurs in rounded crys- 

 talline masses or imperfect crystals from one tenth to one half an 

 inch in diameter, associated with a white or greenish-white crys- 

 talline feldspar, black augite, a little brown mica, and magnetite. 

 The proportion of olivine is very variable, but in some parts it 

 is the predominant mineral. Its color is olive-green, passing into 

 amber-yellow. The grains,which are translucent, are much fissured 

 and very brittle. The pulverized olivine gelatinizes with chlorhy- 

 dric acid in the cold, and is almost instantly decomposed when 

 warmed with sulphuric acid diluted with its volume of water, the 

 silica separating chiefly in a flocculent form, and enclosing small 

 grains of the undecomposed mineral, which are left when the 

 ignited silica is dissolved by a solution of soda. A little silica is 

 however retained in solution, and is precipitated by ammonia 

 with the oxyd of iron. Two analyses of different portions of the 

 olivine made in this way gave, after deducting the undecomposed 

 minciral, the following results : 



