238 A. E. Barlow — Relations of the Laurentean, etc. 



green hornblende and brown biotite. The foliation is produced 

 by the approximate parallelism in the cleavage directions of the 

 minerals. The only other constituents visible under the micro- 

 scope are quartz and ilmenite surrounded by veins of sphene 

 (leucoxene). Of the origin of this rock we can say nothing 

 now with certainty. It may well have resulted from the ex- 



Fio-. 2. 



treme metamorphism of some basic eruptive, but from a small 

 specimen like this it is unsafe to draw any such conclusion. 

 The feldspathic vein is a much coarser aggregate of quartz, 

 orthoclase and plagioclase with a little green hornblende. The 

 feldspar has many minute hornblende needles secondaril} 7 devel- 

 oped in it, but otherwise the rock appears like a fresh granite." 



The eruptive origin assigned to this rock by Prof. Williams 

 would seem to agree with its relations in the field and on ac- 

 count of its intimate association with granitoid gneisses we 

 have classed it with the Laurentian and so colored it on the 

 recent maps issued by the Canadian Survey of the Sudbury 

 Mining District. These gentlemen also make me state that I 

 " drew the conclusion that nowhere on the north shore of Lake 

 Huron are any detritals which are later than and rest uncon- 

 formably upon the basement complex," when in the article is 

 distinctly described (see p. 37, American Geologist, Vol. "VI, 

 1890) a thinty bedded slate conglomerate resting upon the up- 

 turned edges of the Laurentian gneiss on Annima-Nipissing 

 Lake. 



Briefly then, the Huronian System may be regarded as the 

 oldest series of sedimentary strata of which we have at present 

 any knowledge in this district, while the Laurentian gneiss or 

 basement complex is the original crust of the earth or the firm 

 floor on which the first sediments were laid down. The com- 

 position of this floor, judging from the overlying strata, would 

 be closely analogous to granite and the laminated pebbles in 

 the slate conglomerates indicates that this basement had. in 



