144 THE VERMILION IRON-BEARING DISTRICT. 



ill a limited development, in the rocks of the areas mentioned, and that 

 its occurrence has simply been overlooked, or else that the spherulites 

 were not recognized as such, but were called amygdules, as they were in 

 the notes of some of the observers in the Vermilion district before attention 

 was called to the true character of these bodies. 



THE ELLIPSOIDAL" STRUCTURE. 



The ellipsoidal structure in pre-Cambrian greenstones (metabasalts) 

 was described by the author in 1899,* and the attempt was made to account 

 for its occurrence. It was concluded that the greenstones correspond in 

 general characters and in mode of origin to aa lava, and that the ellipsoids 

 were due to the breaking up of this relatively Adscous lava. It was 

 concluded that the shape of the ellipsoids was determined to a great extent 

 by the rolling over and over of these units and the pressure under which 

 they existed at this time as well as their cooling and consequent .contraction, 

 with possibly, as an additional and less important factor, the pressure to 

 which they were subjected subsequent to their complete cooling. In a 

 recent article Gregory describes the ellipsoidal structure in Maine andesites, 

 and writes of the brecciated rock (glassy and stony) which fills the 

 interstices between the ellipsoids." This is confirmatory of my statement 

 that the matrix between the ellipsoids in the Lake Suj^erior region was 

 originally a breccia, in part at least, which is now, however, as the result 

 of pressure, almost always distinctly schistose. The conclusion as to this 

 original brecciated character of the matrix was reached chiefly as the 

 result of microscopic study of thin sections of the schistose matrix.'' A 

 clastic matrix has been observed filling the interstices of rocks with similar 

 ellipsoidal structure and is found described in the work of Geikie on the 



"The advantage of using the term " elhpsoidal," ajiplied to designate the peculiar parting found 

 in some of tlie basic igneous rocks, was emphasized in the description of the Crystal Falls iron-bearing 

 district of Michigan : Mon. U. S. Geol. Survey Vol. XXXVI, 1899, pp. 112-124. The writer has observed, 

 in conversation with various geologists, that in practically every case in which the term "spheroidal" 

 was applied to this parting, the person using it had the idea that it corresponded very closely to the 

 secondary spheroidal parting which is so well known in all igneous rocks. Very naturally confusion 

 is thus caused in the minds of the geologists by the use of the term spheroidal to designate the original 

 parting in the rocks on the one hand, and the structure produced as the result of weathering processes 

 on the other. It seems to me, therefore, more than ever necessary to confine the term ellipsoidal to 

 this original parting and the term spheroidal to the structure produced by weathering. 



bUon. U. S. Geol. Survey Vol. XXXVI, 1899, pp. 112-124. 



cAm. Jour. Sci., 4th series. Vol. VIII, 1899, p. 367. 



'^Loc. cit. 



