34 THE CANADIAN NATURALIST. [Feb. 



rites often assume atrachytic form, — the trachy-dolerites already 

 mentioned, — or constitute the lavas from modern volcanoes. 



Among the compound rocks which are related to the preceding 

 group by the presence of aiigite, maybe noticed nepheline-dolerite, 

 in which nepheline replaces the feldspar; and analcimite, a variety, 

 into which analcime enters in large amount. Scapolite also in 

 some cases replaces feldspar,, and forms with green pyroxene, a 

 peculiar aggregate associated with the Laurentian limestones. 

 Leucite enters as an important element in some dolerites, and even 

 replaces wholly the feldspathic element, giving rise to what has 

 been called leacitophyre or leucilite. 



[Leucite is generally regarded as an exclusively volcanic mineral ^ 

 but according to Fournet, it occurs like other feldspars in mineral 

 veins, forming the gangue of certain auriferous veins in Mexico 

 (Geologic Lyonnaise, page 261). According to Scheerer, leucite 

 also occurs in drusy cavities with zeolites and quartz at Arendal in 

 Norway ; although it would seem to be rare in this locality since 

 Durocher was not able to detect it. (Annales des Mines [4], i, 218), 

 The conditions required for the formation of this feldspathide 

 must be peculiar, since the volcanic rocks which afford it are con- 

 fined to a few localities ; and since while it contains a large amount 

 of potash it is a basic silicate, and found among highly basic rocks^ 

 in which potash compounds are generally present only in very 

 small quantities. The agalmatolite rocks, including dyssyntribite 

 and parophite (Geology of Canada, page 484), are however 

 basic aluminous silicates in which potash predominates, and might 

 be supposed under certain conditions of metamorphism to yield 

 leucitic rocks.] 



Thename of melaphyre, which is employed by many writers on 

 lithology requires a notice in this connection. It was proposed by 

 Brongniart as a synonym for black porphyry (mela-porphyre),. 

 and defined by him in 1827 as a porphyry holding crystals of 

 feldspar in a base " of black petrosilicious hornblende." (Classif. des- 

 Roches, page 106.) Subsequent researches showed that some of 

 these porphyries were really augitic ; and Von Buch employed the 

 name of melaphyre as synonymous with augite-porphyry, in which 

 he was followed by D'Halloy. (Des Roches, p. 75.) In consequence 

 of this confusion, and of the vague manner in which the term is 

 used to include rocks which are sometimes dioritesand sometimes 

 varieties of dolerite or basalt, Cotta seems disposed to reject the; 



