168 THE VERMILION IRON-BEARING DISTRICT. 



form of segments of circles. It is evident that tliey did not all begin to 

 form at just the same time or else their rate of growth was not just the 

 same, for if their origin were simultaneous and their rate of growth equal 

 we would get in cross section through such masses a structure resembling 

 that of a honeycomb. These spherulitic greenstones are both fine and 

 coarse grained, the ellipsoidal and spherulitic portions being continuous 

 with the nonellipsoidal and nonspherulitic greenstones. It would seem 

 that tlie ellipsoidal and spherulitic portions of the greenstones represent the 

 surface of greenstone lavas which are to be considered as effusive sheets or 

 flows. These greenstones are cut by dikes of granite-porphyr}^. 



One mile north of North Twin Lake, at the northeast corner of sec. 12, 

 T. 63 N., R. 10 W., these ellipsoidal spherulitic greenstones continue from 

 south of Jasper Lake in almost continuous exposures eastward along the 

 south section line of sec. 6, T. 63 N., R. 9 W., almost as far east as the south 

 quarter post of that section. Here the ellipsoids reach a diam-eter of 3 to 

 4 feet and some of them are solid masses of spherulites, each sphendite 

 showing its radial structure very beautifully on the weathered surface. 



About 400 paces north, 100 paces west from the southeast corner of 

 sec. 35, T. 62 N., R. 14 W., interbedded amygdaloidal and ellipsoidal basalts 

 occur. They are all somewhat schistose. As we continue southward 

 studying the exposures, Ave find that they show an increasing degree of 

 metamorphism. They finally become amphibole- and mica-schists and 

 gneisses, and but for the presence of the elongated ellipsoids and the 

 amygdules, filled with chlorite and pinkish quartz, one could not be sure, 

 from the field study, of their igneous origin. The rocks in this locality 

 resemble in a striking deg'ree the crystalline schists occurring in the 

 vicinity of Bone Lake", in the Crystal Falls district of Michigan. 



It has been stated repeatedly that certain of the greenstones possess 

 an amygdaloidal structure, and that with these tuffs are associated. These 

 facts have been cited as evidence of the volcanic nature of the greenstones. 

 Such associated and presumably interbedded tuffs and amygdaloidal and 

 porphyritic greenstones are very well exposed upon the west and north 

 west slopes of a high hill in the northwest quarter of sec. 19. T. 64 N., 

 R. 10 W. Here the greenstones, which are both fine and coarse grained, 



«The Crystal Falls iron-bearing district of Michigan: Men. U. S. Geol. Survey Vol. XXXVI, 

 1899, pp. 148-152. 



