176 THE CANADIAN NATURALIST. [June, 



fissile and schistose structure, which gives, at first sight, the aspect 

 of stratification to what is undoubtedly an intrusive rock. When 

 exposed to the action of the waves on the lake-shore, its structure 

 appears to be columnar, and sometimes concretionary. This rock 

 is described as composed of a reddish or pale leek-green compact 

 feldspar, holding crystals of the same mineral. (Geology of New 

 York, vol ii, page 84.) These intrusive feldspathic rocks on Lake 

 Champlain resemble closely the trachytes of Montreal and Cham- 

 bly, — with the latter of which, the trachyte of Shelburne, the 

 only one of them which has been chemically examined, closely 

 agrees in composition. 



DOLERITES. 



r 



The anorthosites, which yet remain to be described, may be 

 divided into two groups, — those composed of anorthic feldspars with 

 augite, constituting the dolerites, and those in which similar feld- 

 spars are associated with hornblende. The general geognostical 

 relations of these two groups of rocks in the districts under dis- 

 cussion have already been indicated. 



Grenville. — It has already been stated on page 163 that the 

 oldest known intrusive masses which traverse the Laurentian 

 series are of dolerite, and that the dykes of this rocks are inter- 

 sected by the syenite, which was succeeded by the orthophyre or 

 quartziferous porphyry, Nothing corresponding to the syenite or 

 the orthophyre is met with among the adjacent Lower Silurian 

 strata, which are seen to repose upon the worn surfaces of these 

 intrusive rocks. A fourth series of dykes of a porphyritic dolerite 

 is however found to cut all of the preceding rocks, and is perhaps 

 identical with some of the dolerites which intersect the Silurian 

 rocks of the island of Montreal. In the other parts of the Lau- 

 rentian series, so far as yet examined, intrusive rocks have been but 

 seldom met with.. Much of what has been called syenite and 

 granite in various parts of the Laurentian region, seems, like the 

 hypersthenite and other anorthosites of the Labrador series, to be 

 indigenous. 



The dykes of this most ancient dolerite or greenstone in 

 Grenville, have a well-marked columnar structure at right angles 

 to the plane of the dyke. They are fine grained, dark greenish- 

 gray in color, and weather greyish-white. Under a lens, the rock 

 is seen to consist of a greenish- white feldspar with a scaly fracture. 



