202 i?. A. Daly — Secondary Origin of Certain Granites. 



cia observed along its contact with them. If, however, the con- 

 tact belt is examined very closely, it is found that although the red 

 rock is always accompanied by a zone of this belt, there are 

 localities in which the latter occurs without the presence of the 



former The metamorphosing rock seems to be the gabbro. 



Just as in the case of the contacts with the red rock, the quartz- 

 ites become mottled as they approach the eruptive, and inclu- 

 sions of the former in the latter are so frequent that there appears 

 to be a gradual transition between the two rocks."* 



Similar shatter-breccias are described at the northern con- 

 tact. The metamorphism of the inclusions is there the same 

 in kind as on the southern contact but is less intense. f 



(b) A significant discovery was made at a mining prospect 

 on Governor's Island just south of Pigeon Point. The shaft 

 started in hardened slate at the surface, then struck red quartz- 

 ite and finally red granite where the sinking was discontinued. 

 In this case there is no question that the sediments overlie the 

 granite in a relation similar to that involved in the sill theory 

 of the Pigeon Point intrusive.;); 



(c) Parallels to the Pigeon Point case have also been found 

 on Spar, Jarvis and Victoria Islands. § Lawson has described 

 other examples among the Logan sills of Lake Superior, and 

 says that the sills are repeated by step-faults gently tilting the 

 sills to the southeast at the maximum angle of Hve degrees. || 



id) Grant describes the great gabbro area of Cook County, 

 Minnesota, as a laccolith in the gently dipping Animikie quartz- 

 ites, slates and graywackes. He maps soda granites passing 

 into alkaline quartz porphyries on the southern flank of the 

 gabbro or in it. This latter occurrence is possibly to be related 

 to the occasional horizontal dips of the Animikie.^f 



N. H. Winchell maps a broad band of the red granite to the 

 southward of the huge gabbro mass of Lake County. He 

 states that the southern limit of the gabbro forms the northern 

 limit of the red granite, but that there are numerous places 

 where these rocks are intricately interbedded and in some 

 instances isolated areas of the red rock are surrounded by 

 gabbro.* * The official atlas of the Minnesota Geological Survey 

 indicates still other large-scale examples of the same or similar 

 close relations of gabbro and red granite — notably those mapped 

 in vol. vi, plates 68, 69, 84, 85 and 87. 



*Op. cit., pp. 28-29. f Op. cit., p. 81. 



X Final Report, Geol. Surv. of Minnesota, vol. iv, 1899, p. 516, and vol. v. 

 900, p. 799. 



§Bayley, op. cit., p. 30 ; cf. E. D. Ingall, Ann. Eep. Geol. Surv. Canada, 

 1888, Pt. H, pp. 45 and 49. 



|j Bull. 8, Geol. Surv. Minnesota, 1893, pp. 30-33-42-44. 



*\ Final Rep. Minn. Geol. Surv., vol. iv, 1899, pp. 323 and 326. 



**Ibid., pp. 296-7. 



