DISTRIBUTION BRITISH AMERICA 613 



or pillow parting, often ascribed to subaqueous cooling, indicating substantially 

 shore or shallow water conditions." 



These are overlain iinconformably by a series of limestones, sand- 

 stones, and quartzites of the Nastapoka group. 



"Then came the great outburst of vulcanism . . . surface basalts, giving 

 evidence by the thinly bedded, ripple-marked sediments above and below and 

 between them, and by their own combination of ellipsoidal with basaltic, 

 amygdaloidal, and scoriaceous textures, of extrusion at or near water level, 

 not improbably along tidal flats. . . . 



"The basalts are all characterized by an abundance of jasper veins, jasper 

 filling the amygdules, jasper cement between the blocks of ellipsoidally parted 

 flows. Striking indeed is the brightly colored jasper cement outlining the 

 pillow forms on many of the beautifully exposed glaciated surfaces." 



UNITED STATES 



Lake Superior region. — Williams^^ fonnd some of the greenstone 

 schists of the Menominee and Marquette districts characterized by divi- 

 sion into oval or lenticular areas which interlace and are separated by 

 finely schistose material of much finer grain, at first glance resembling 

 the spheroidal weathering of many eruptive rocks. The spheroidal, 

 ovoid, lenticular, and more irregularly shaped masses, differing much in 

 size and form, often exhibit a tendency to fit together like stones in a 

 mosaic, but everywhere separated by interlacing bands of softer, more 

 schistose, and generally darker material, which winds about the massive 

 cores, becoming thinner and thicker as the masses approach each other 

 or are more widely separated by the rounding of corners (compare plate 

 17, figure 2). This is not an agglomerate or a concretionary structure. 

 Possibly in some cases it may be due to contraction which has produced 

 spheroidal parting, the author suggests, like perlitic structure on a large 

 scale; but on the whole he regards Kothpletz^s explanation of similar 

 structures in central Saxony as the true one — that is, the rock masses, 

 already finely subdivided by joint cracks, have had the corners of the 

 individual blocks rounded and the interstitial matter produced by the 

 rubbing of the blocks together under intense orographic pressure. 



WinchelP^ described similar ellipsoidal greenstones from Ely, Minne- 

 sota (plate 17, figure 2), as agglomerates, which he believed were pro- 

 duced by the. falling of volcanic bombs into the sea. He observed that 

 about the peripheries of the masses there are numerous radial tubes, 



^^ G. H. Williams : The Greenstone schist areas of the Menominee and Marquette 

 regions of Michigan. Bull. U. S. Geol. Survey, No. 62, 1890, pp. 137, 106-108, 203, 204. 



«» N, H. Winchell : The Kawishiwin agglomerate at Ely, Minn. Am. Geol., vol. ix, 

 1892, pp. 359-368, 



