SUDBURY NICKEL-BEARING ERUPTIVE 551 



The last paper of the morning session was the following : 



S UDB UR Y NICKEL-B EA RING ER UP TI VE 

 BY A. P. COLEMAN 



I Abstract'] 



It has long been known that the deposits of nickeliferous pyrrhotite of the Sud- 

 bury region, Ontario, are associated with bands of a peculiar eruptive rock, gener- 

 ally called diorite, but really norite, passing gradually into micropegmatitic granite. 

 Two ranges have been distinguished by prospectors, a southern and a northern, the 

 best known ore deposits being at the basic margin of the southern range or on 

 dike-like offsets from it. Daring the past summer work carried on for the Bureau 

 of Mines of Ontario has proved that the two ranges are joined at the ends, form- 

 ing an oval basin 37 miles long and 15 miles wide. The band of eruptive rock, 

 which has a width of from 1 to 4 miles and averages nearly 2 miles in thickness, 

 dips inward on all sides and evidently forms a synclinal trough inclosing an area of 

 pyroclasticand ordinary sediments of doubtful age (Cambrian or Upper Huronian). 

 Both the upper and lower sides of the synclinal sheet are in eruptive contact with 

 the adjoining rocks, the acid edge with the sediments just mentioned, the basic 

 edge with Laurentian gneiss or Huronian quartzites or schists. 



Taken as a whole, the eruptive sheet is of a laccolithic character, but unlike any 

 other known laccolith in shape and dimensions, since it probably includes at least 

 600 cubic miles of rock. The ore bodies are often large, and one mass, the Creigh- 

 ton mine, contains more than 3,000,000 tons of pyrrhotite. Those at the basic 

 edge of the sheet blend into the general mass of the norite and must be looked on 

 as due to magmatic segregation, perhaps by gravity, as they are all found in bay- 

 like depressions of the country rock on the outer or lower side. The ore bodies 

 on offsets are partly formed by cooling from the molten condition, but generally 

 show evidence of the action of circulating waters, and may pass into ordinary vein 

 deposits not evidently connected with norite. 



The synclinal form of the sheet is supposed to be due to the sinking of the sub- 

 stratum owing to the removal of molten material from beneath. The observed 

 inward dip of the lower surface of the sheet averages about 45 degrees, and the 

 thickness of sedimentaries included within the syncline shows that the center 

 must be several thousand feet below the surface. 



Professor Coleman's paper was discussed by the President, G. K. 

 Gilbert, and W. H. Hobbs. 



The Society adjourned for the noon recess. 



At 2 o'clock p m the Society reconvened, and the first paper read was 

 the following : 



WIDESPREAD OCCURRENCE OF FAYALITE IN CERTAIN IGNEOUS ROCKS OF 



WISCONSIN 



BY SAMUEL WEIDMAN 



[Abstract] 



In the central part of Wisconsin, within the area of pre-Cambrian rocks, is a 

 large variety and abundance of igneous rock which intrudes a much older sedi- 



