On Some Dykes Contaimm; Huromte. 37 



and shows considerable " bleaching " and chloritization. The larger 

 phenocrysts, which are probably labradorite, are mostly quite fresh and 

 glassy, but irregular areas are more or less clouded by the development 

 of minute scales of sericite or kaolin. The rock differs from the 

 Temagami specimen in the absence of olivine and the advanced 

 uralitization of the augite 



3. Locality. Landing at Silvei Islet, north shore of Lake Superior. 



Prof. Winchell thus describes this rock (601) : (1) '-A coarse porphy- 

 ritic 'dioryte'in a dyke running parallel to and contiguous, to and 

 passing into (602) a fine grained ' dioryte ' in the form of a dyke. The 

 interval of transition is perhaps two feet wide, and the crystals of felspar 

 are scatteringly disseminated through it on the south side, and wholly 

 disappear on the north side. They run in the same direction a.s the 

 dyke on Silver Islet. The whole is 45 feet wide, but is evenly divided 

 between Nos. 601 and 602 from about a mile north of the ' Landing at 

 Silver Islet."' 



The thin section under the microscope showed an aggregate of 

 plagioclase (labradorite), augite, serpentine and opaque iron ore. 

 The phenocrysts of plagioclase as well as the lath-shaped crystals pre- 

 sent in the groundmass show more or less 'cloudiness' due to the 

 development of minute scales of muscovite. Irregular fissures travers- 

 ing the felspar are filled with yellowish green serpentine derived from 

 that present in the surrounding matrix. The augite, which is quite 

 fresh, has a reddish colour, and is slightly pleochroic. It occurs in 

 irregular grains and areas filling in the spaces between the plagioclase 

 laths. The yellowish green serpentine, which is abundant, is present 

 in areas whose external form and internal arrangement at once suggest 

 its alteration from olivine, which was no doubt originallv present. 

 These phenocrysts of labradorite are much fresher than those to which 

 the name " Huronite ' : has usually been applied, but under similar 

 conditions of alteration there is no doubt that they would become so 

 decomposed as to be indistinguishable from this mineral. 



(1) Specimens Nos. 601 and 602, ioih Annual Report of Geological and Natura 

 History Survey, Minnesota, page 56. 



