20-i R. A. Daly — Secondary Origin of Certain Granites. 



and 17 miles wide, and probably a mile and half to two miles 

 thick on the average, if the (average centripetal) dip (of the 

 sheet) is 45 degrees."* 



This great sheet cuts sediments and schists referred to the 

 Lauren tian and Upper Huronian. Their general field relations 

 are summarized in the diagrammatic map of fig. 5, drawn from 

 a part of Coleman's official map of the " Northern Nickel 

 Range." Again the gabbroid rock (norite) is seen to be con- 

 centrated on the lower contact of the sheet, the acid rock, 

 micropegmatite or granophyre, graduating into true granite, on 

 its upper contact, while between the two is a zone of inter- 

 mediate rock. On the " Southern Nickel Range " across the 

 spoon-shaped basin, Barlow has determined the same arrange- 

 ment of acid, intermediate and basic zones in the sheet, which 

 there, however, agreeably with the basin theory of structure, 

 has a northerly dip ; so that in this case, the norite occurs on 

 the south side of the sheet, the granite-gran ophy re zone on its 

 northern side. On the basin theory of the structure, the 

 volume of the granite-gran ophyre in this sheet is to be meas- 

 ured by hundreds of cubic miles. 



All around the basin the nickel ores form a more or less 

 continuous zone at the lower contact of the norite. The sul- 

 phides are also to be found in especial abundance as segrega- 

 tions in apophysal offshoots of the norite where the basic magma 

 penetrated fissures outside the lower contact of the sheet. 



Coleman states that where the band of eruptive (outcrop 

 edge of the sheet) is narrow, there is less change in the rock in 

 passing from the lower to the upper contact, the most basic 

 norite as well as ore being absent for the most part. He also 

 notes the absence of granophyre or granite in the smaller 

 intrusions of the norite which occasionally appear outside the 

 main basin. f 



Eruptive breccias due to the shattering of the invaded forma- 

 tions by the hot magma are found at both upper and lower 

 Contacts.;); 



Coleman notes that in the northern nickel range the con- 

 tact metamorphism is more intense next the upper acid zone 

 than next the norite. He explains this as possibly due to the 

 fact that the rocks at the lower contact were already well crys- 

 tallized before the intrusion took place, while the sediments 

 along the upper contact were then capable of notable minera- 



*A. P. Coleman, Eep. Bureau of Mines, Ontario, 1903, p. 277. Cf. A. E. 

 Barlow, Ann. Rep. Geol. Surv. Canada, vol. xiv, 1904, p. 72 ; also stereogram 

 accompanying Coleman's report. 



f 1904 report, p. 212, and 1903 report, p. 286. 



JA. E. Barlow, op. cit., pp. 122, 129 and plates; A. P. Coleman, Rep. 

 Bureau of Mines, Ontario, 1904, p. 213 ; T. L. Walker, Quart. Jour. Geol. 

 Soc, vol. liii, 1897, p. 54. 



