612 J. v. LEWIS ORIGIX OF PILLOW LAVAS 



feet. The ovoid masses are not in close contact but are separated from one 

 another by an interstitial material. The concretionary masses are, at their 

 points of nearest approximation to one another, generally about a half an 

 inch or an inch apart, no matter what may be their size, so that when the 

 ovoid masses are large the interstitial material appears in sections as thin 

 anastomosing sheets in which is developed a schistosity parallel to the outlines 

 of the ovoid masses they enclose. The interstitial filling is generally of a 

 darker color, more chloritic. softer, and a finer, more homogeneous texture than 

 the ovoid masses and weathers out often leaving the latter, in the sections 

 afforded by glaciated surfaces, surrounded by shaip little trenches. , . . 

 ''The ovoid masses are uniformly arranged as regards the direction of their 

 long axes, and each one is surrounded by a shai^ border, half an inch wide, 

 of a dark greenish-gray color, which has been more resistant to weathering 

 agencies than the rest of the rock." 



In portions of the greenstone that have been metamorphosed into horn- 

 blende schist the interstitial matter forms thin anastomosing sheets of 

 dark-green soft chloritic material, some of ^^'hich lose themselves in 

 tapering disconnected fashion in the main mass of the rock. 



Willmott'^^ found the greenstone schists of the Michipicoten area char- 

 acterized in many places by the same **elliptical" structure or "ellipsoidal 

 parting," as described by Clements in the Hemlock formation of the 

 Crystal Falls district and in the greenstones of the Vermilion district. 

 They are regarded as undoubted lava flows, but the origin of the struc- 

 ture is not discussed. 



Exactly similar greenstones have been described by Allen^® from the 

 Woman Eiver district, in the Sudbury Mining Division of Ontario. 

 These and similar greenstones of the Lake Superior region in general 

 are thought to be of subaqueous origin. 



Hudson Bay. — In an "Algonkian basin" along the southeast shore of 

 Hudson Bay. where the formations resemble in many resj^ects those of 

 the Lake Superior region, Leith®" interpreted the ''ellipsoidal or pillow 

 parting^^ in basalts of the Eichmond group as evidence of subaqueous 

 eruption. 



•'These were poured out approximately along the shores and tidal flats, for 

 the coarse sediments immediately below and above them exhibit the wide 

 variety of ripple and current marks duplicated in the tide flats of the shore 

 today. Also the flows themselves show not only basaltic, amygdaloidal. and 

 scoriaceous textures resulting from surface cooling, liut the peculiar ellipsoidal 



^ A. B. Willmott : The Michipicoten Huronian area. Am. Geol.. vol. xxviii. 1901. pp. 

 14-19 : also The nomenclature of the Lake Superior formations. Jour. Geol.. vol. x. 1902, 

 pp. 67-7G. 



®" R. C. Allen: Iron formation of Woman River area. Eighteenth Rept. Ontario Bu- 

 reau of Mines. 1909. pt. i. pp. 254-262. 



^ C. K. Leith : An Algonkian Basin in Hudson Bay : A comparison with the Lake 

 Superior Basin. Econ. Geol.. vol. v. 1910, pp. 227-246. 



