Professor Emerson on HalVs Geological Collections. 5ft 1 



orthoclase, gray i)lagioclasG and quartz much more coarsely crystallized than in 

 the mass of the rock. 

 45. Magnetite gneiss. 



A fine-grained granitoid gneiss, having at first sight somewhat the appear- 

 ance of andesite. The pearl-gray ground mass is a fine-grained mixture of quartz 

 and a feldspar, mostly triclinic, and scattered in this abundantly are grains of 

 magnetite, and sparingly brown decomposed biotite. 



French Head, Field Bay. 

 40. Magnetite gneiss. 



A decomposed granitoid gneiss, closely resembling 43. This occurs in sev- 

 eral large pieces. 



47. Magnetite gneiss. 



Contains orthoclase, albite, and quartz in about equal quantities, less abun- 

 dantly magnetite, and as a product of alteration chlorite. The rock is fine- 

 grained, fresh, pale flesh-colored, mottled with spots of dark green color, consist- 

 ing of magnetite and chlorite, which lie in the x)lane of stratification. The chlorite 

 fills also as thin seams a system of cleavage cracks passing at large angle to the 

 cleavage. 



Examined in thin section, the feldspars are for the most part water clear, 

 showing only incipient clouding of kaolin on fissures, and extremely delicate and 

 minute dentritic infiltrations of ochre. The albite is predominant, and here and 

 there grown together with orthoclase. The quartz contains in immense numbers 

 small round and large irregular and contorted fluid inclosures, with very large 

 bubbles, moving only when heated. The magnetite grains — J to 1 mm. in 

 diameter— are surrounded by a ring of bright green plates of chlorite, and from 

 these as centers the chlorite passes outward in the fissures, producing the patches 

 of green color. 



Locality, Frobisher Bay. 



48. Magnetite gneiss. 



A fine-grained granitoid mixture of quartz, orthoclase, and sparingly a 

 triclinic feldspar, to which granular magnetite, arranged in parallel blotches, gives 

 a rudely gneissoid structure. This and the foregoing agree exactly with the Lau- 

 rentian gneisses from Grenville, Canada. 



Labeled, " From various places up Bay of Frobisher and near head of it." 



49. Graphitic gneiss. 



Two large and many small specimens of a decomposed and rusty granitoid 

 gneiss of a grey color when fresh. It contains a dark-brown mica, minute crystals 

 S. Ex. 27 36 



