. Bulletin of the Geological Society of America. Geology. 230 A. Y. COLEMAN—CLASTIC HURONIAN ROCKS. Of the smaller areas mention may be made of Sultana island a few miles from Rat Portage, famous for its gold mine. Here a boss of coarse porphyritic granitoid gneiss a mile in length by half a mile in width presents the same eruptive contact with the green Keewatin rocks as one finds around the larger masses. Another similar boss of coarse Lauren- tian granite was found by the writer at Caribou lake, east of the lower end of lake Manitou, the area being only about a square mile. Exam- ples of in
. Bulletin of the Geological Society of America. Geology. 230 A. Y. COLEMAN—CLASTIC HURONIAN ROCKS. Of the smaller areas mention may be made of Sultana island a few miles from Rat Portage, famous for its gold mine. Here a boss of coarse porphyritic granitoid gneiss a mile in length by half a mile in width presents the same eruptive contact with the green Keewatin rocks as one finds around the larger masses. Another similar boss of coarse Lauren- tian granite was found by the writer at Caribou lake, east of the lower end of lake Manitou, the area being only about a square mile. Exam- ples of intermediate sizes may be found on the Canadian Geological Sur. vey's maps of the region. Finer grained granites, generally foliation on the edges of the areas, are common also both in Huronian strips and in the Lauren- tian; and many more small knobs and bosses than were mapped by Lawson in his somewhat hurried work will be found from time to time; such as the area of protogine containing so many gold-bearing veins at. LAURENTIAN G/)ANIT£. Scale or Miles, Figure l.—Gfeologicai Map of Part of Western Ontario. Shoal lake. So far as known, these granite bosses are later than both Keewatin and Laurentian, having penetrated both. It is, however, not always easy to say whether a given rock is Laurentian or a later granite, and it is likely that the two are connected in origin and might be ar- ranged as a consecutive series. Both Laurentian and other granites send off felsitic dikes into the adjoining rock, and in this way one may often discover the proximity of a gneiss or granite area a quarter of a mile before reaching the contact. It is worthy of note, as observed by Lawson, that often the gneiss grows darker and more basic near the contact with basic Huronian rocks, as though some of the latter material had been incorporated in the un- derlying Laurentian. The accompanying map, figure 1, will illustrate the geological relation- ships just mentioned. It has been pan
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