OF CENTRAL CANADA PART III. 191 



somewhat in the composition of its feldspar, and becoming strongly 

 hornblendic, passes into a variety of diorite ; but the distinction 

 between granitic trachyte and diorite is in many cases purely arti- 

 ficial. 



5. Obsidians and Pitchstones : This division includes lavas and 

 other rock matters of igneous origin which occur in a more or less 

 vitreous or glassy state, and present an essentially feldspathic compo- 

 sition. The term obsidian is usually restricted to grey, green, brown, 

 or black rocks of this character, occurring in actual connection with 

 volcanoes, whether active or extinct. A variety containing small 

 spherical secretions of a somewhat pearly aspect is known as Pearl- 

 stone. These rocks break with sharp edges, and the fractured sur- 

 face shows conchoidal markings. Pitchstone occurs chiefly in the 

 form of dykes in trappean districts. It is mostly of a black colour, 

 and pitchlike or resinous aspect, but some varieties are dull-green, 

 grey or red. A porphyritic variety, traversed by small veins of 

 agate, occurs near the deserted copper workings on the Island of 

 Michipicoten, Lake Superior ; and some of the dykes and bedded 

 traps near Michipicoten Harbour on the mainland, appear to be 

 intermediate in character between pitchstone and ordinary basalt. 

 Although not recognized in Ontario or Quebec, examples of obsidian 

 are not uncommon in British Columbia. 



6. Lavas : These comprise the actual rock-matters which issue in 

 a molten condition from volcanoes. They present vesicular, compact, 

 columnar, porphyritic, and other varieties, and are of two general 

 kinds as regards composition : feldspathic, and feldspatho-augitic. 

 The first, and by far the more common of the two, are composed 

 essentially of feldspar, and are mostly of a light or dark grey colour. 

 They pass into trachytes. The second, composed essentially of feld- 

 spar and augite. are dark-ijreen or black in colour and are undis- 

 tinguishable, except by their actual conditions of occurrence, from 

 many traps and greenstones. Examples of the group, as thus 

 defined, are unknown within the limits of Ontario and Quebec, but 

 occur in British Columbia. 



V. MINERAL VEINS. 



In a review of the characters and conditions of occurrence of rock- 

 masses, the subject of mineral veins cannot be altogether passed over, 



