THE SILICIFIED GLASS-BRECCIA OF VERMILION RIVER, 

 SUDBURY DISTRICT. 



BY GEORGE H. WILLIAMS. 



[Read before the Society December 31, 1890, as an Appendix to the communication 07i 

 the Nickel and Copper Ores of Sudbury District, Canada, by Dr. Robert Bell.) 



Among a considerable series of rocks from the Sudbury district which I 

 have recently had the pleasure of microscopically examining for Dr. Robert 

 Bell, of the Canadian Geological Survey, there was one of such unusual 

 petrographical character that it well merits a special description. More- 

 over, this rock is not merely a petrographical anomaly, but it also occupies 

 so large an area as to become geologically of great importance as a member 

 of the Sudbury series. 



The specimens of this rock examined by me in Dr. Bell's collection bear 

 the label ''Lowest fall of Onaping river, Sudbury," and turn out to be 

 nothing less than a breccia composed of sharply angular fragments of vol- 

 canic glass and pumice, which, in spite of almost complete silicification, still 

 preserve every detail of their original form and microlitic flow-structure with 

 a distinctness not to be exceeded by the most recent productions of this kind. 



Such porous glassy rocks are well known to be more subject than any 

 others to either alteration or complete removal, so that the preservation of 

 this glass-breccia from Huronian times without loss of its original char- 

 acteristics must be regarded as very exceptional. The production of a mass 

 like this on so large a scale in any geological time is also a matter worthy of 

 notice, for Dr. Bell has traced it as a wide band for over forty miles without 

 then reaching its northern limit. 



The following memoranda on the occurrence and distribution of this 

 ancient glass-breccia have been furnished by Dr. Bell and may best be given 

 in his own words : 



"This remarkable rock lies aloni^ the northwestern side of the Huronian trough, 

 having the red quartz-sj'enite, which may be Laurentian, on its northwestern flank 

 and being bounded on the southeast by what is here the highest member of the series, 

 which consists of thick-bedded dark bluish-gray argillaceous sandstone, full of clear 

 grains of quartz and interstratified with shaly beds of the same color, all overlain by 

 black shites. Towards its southwestern termination the breccia itself passes into a 

 black slaty mass holding many pebbles, mostly of syenite. 



" The belt runs from the township of Trill northeastward along the northwestern 

 side of Vermilion river to a point opposite Wahnapitae lake, where it cuts across the 

 river and continues on northeastward ; but its limit in that direction has not been 

 accurately ascertained beyond forty miles from the township of Trill. In this town- 



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