1868.] MACFARLANE — GEOLOGY OF LAKE SUPERIOR. 243 



described by me in this journal.* The delessite which enters 

 so largely into their composition can scarcely have been one 

 of the original constituents, and has probably resulted from 

 the gradual alteration of augite, since authenticated instances 

 are on record of the conversion of that mineral into delessite and 

 green-earth. The specific gravity of these rocks varies from 2.83 

 to 2.89. When ignited they lose 1.32 to 3.09 per cent, of their 

 weight, the powder changing from light greenish-grey to a light 

 brown colour. Digested with hydrochloric acid from 32.44 to 

 35.72 per cent, of bases are removed from them, the greater part 

 of which belongs to the chloritic constituent. While the variety 

 of melaphyre first above described is seldom found with 

 amygdaloidal structure, the delessitic melaphyres are exceedingly 

 prone to be developed as amygdaloids. In this case the rock 

 contains amygdules of small size but very numerous, and they are 

 either filled with delessite alone, or are lined with a coating or 

 rind of that mineral, in which latter case calcspar generally 

 fills out the centre of the cavity. Quartz or agate is 

 comparatively rare in amygdaloids the matrix of which is 

 delessitic melaphyre. 



Compact Melaphyre. — When the small grained melaphyres 

 above described become so fine-grained as to render the recognition 

 of their constituents impossible, there results the fine-grained traps 

 which are so numerous on the south-west coast of Mamainse and 

 on Michipicoten Island. These rocks vary from reddish, bluish, 

 greenish, or greyish black, to decided black in colour, and possess 

 not unfrequently conchoidal fracture and resinous lustre. Their 

 specific gravities vary from 2.67 to 2.898, and they' fuse before 

 the blowpipe to glasses of black or brownish black colour. 

 Occasionally their material becomes less homogeneous, and presents 

 the appearance of an intimate mixture of reddish grey and green 

 coloured specks, which may perhaps represent partially developed 

 constituents. They exhibit various phenomena as regards 

 divisional joints. Some possess a rudely columnar structure, 

 others have planes of separation forming various angles with the 

 plane of bedding, several shew a tendency to separate into flags, 

 while a few instances are observable of curved shaly separation, 

 (Krwnmiscliaalige Absonderung) . Transitions can frequently be 

 traced from these compact melaphyres to others approaching in 



* Yol. iii., Second Series, p. 2. 



