312 THE VERMILION IRON-BEARING DISTRICT. 



granite debris. The most noticeable differences in the appearance of the 

 Ogishke conglomerate are between those varieties which are made up almost 

 exclusively of greenstone, which are therefore green in color, and those in 

 winch the granite bowlders form the greater portion, such varieties having a 

 gray to reddish color on weathered surfaces. Between these there are, of 

 course, all gradations. In general the gray, bluish- to greenish-gray, slate- 

 colored, and green-colored rocks predominate. These different kinds of 

 conglomerates were noted before they were all correlated, and, for 

 convenience, the conglomerate made up essentially of greenstone pebbles, 

 which is so typically developed in the vicinity of Moose Lake, was spoken 

 of as the greenstone conglomerate. These greenstone conglomerates were 

 for a while rather puzzling, as it was not easy to determine whether they 

 were volcanic tuffs or true sedimentary rocks, in the one case contempo- 

 raneous with and in the other older than the greenstones with which they 

 were associated. Grant, after having studied the district, reached the 

 conclusion that the clastic rocks on the south flank of the Archgan tongue 

 lying south of Gull Lake and extending thence eastward were tuffs derived 

 from these greenstones." Field work having for its object the determination 

 of this particular point has been carried on, and as a result sufficient 

 indications of the sedimentary origin of the elastics have been found to 

 justify their classification as conglomerates. Moreover, in numerous other 

 places elsewhere in the district, interbedding of the finely bedded sediments 

 with these conglomerates and gradations between them have been observed, 

 so that there can be absolutely no doubt that they are normal sediments. 

 The conglomerate at Ogishke Muncie Lake contains pebbles of greenstone, 

 granite, jasper, and other varieties of rocks, and is the normal basal conglom- 

 erate of the Lower Huronian for the eastern part of the Vermilion district. 

 Microscopic characters. — A certain amount of microscopic study was 

 made of the conglomerates. This consisted primarily in the determination 

 of the characters of the pebbles and of the matrix. In addition to the 

 constituents recognizable macroscopically, which have already been enum- 

 erated, the microscope discloses the presence of fragments of basaltic lavas 

 with various microscopically recognizable textures, spherulitic rhyolite, 

 rhyolite-jDorphyry, pieces of quartz and feldspar in pegmatitic intergrowth 



n Geology of the eastern end of the Meaabi iron range in Minnesota, by U. 8. Grant: Engineers' 

 Yearbook, University of Minnesota, 1898, p. 54. 



