614 J. V. LEWIS ORIGIN OF PILLOW LAVAS 



about an inch apart^ having a diameter of about 2 millimeters and a 

 maximum length of about 1 inch. These are normally filled with calcite. 



Winchell"° took exception to the dynamo-metamorphic origin of the 

 Lake Superior greenstones advocated by Williams. While there had been 

 some later pressure and stretching, he questioned the competency of 

 pressure to produce both the parallel banding of the schists and the 

 schistose layers that wind round and separate the spheroidal masses. In 

 a later publication/^ after describing other occurrences of greenstones, 

 which consist of rounded masses, with the "curiously amygdaloidal pe- 

 riphery" of radial pipe-like tubes filled with calcite or chalcedonic silica, 

 the author again reiterates his conviction that they are largely of pyro- 

 clastic origin, but finally concedes that "their nature and origin are 

 problematical." 



Smyth,^^ from a study of the Marquette greenstones, concurs in the 

 opinion of Rothpletz and Williams that the spheroidal structure is of 

 mechanical origin. 



Clements^^ has described fully the ellipsoidal structure in the meta- 

 basalts of the Hemlock formation in the Crystal Falls district. Many of 

 them resemble a conglomerate of rounded boulders, all of the same kind 

 of rock, in a matrix of very small amount and very different color. These 

 are associated with and grade into nonellipsoidal varieties, apparently 

 constituting the surfaces of the flows, and in some cases the whole flow. 

 The masses range from a few inches to 6 or 8 feet in diameter, with 

 sections generally ellipsoidal in all directions, and the longest axes ap- 

 proximately parallel. Some of the ellipsoids "are only half formed — 

 that is, attached by one side to the main unbroken part of the lava flow, 

 the other side showing a rounded outline." 



The amygdaloidal varieties are generally most amygdaloidal at the 

 surfaces of the masses, and some of these are much more so at the top 

 surface. Roughly radial jointing is visible in some of the masses. Some 

 also show a transition into the chloritic matrix, which the author believes 

 is clearly derived from the body of the rock and was probably originally 

 a glass. Many interspheroidal spaces with triangular cross-section are 

 filled with vein quartz instead of schistose material. 



Clements points to the concentric arrangement of the amygdules as 



'" N. H, Winchell : The origin of the Archean greenstones. Geol. and Nat. Hist. Sur- 

 vey of Minn.. 23d Ann. Rept. 1895, pp. 4-35. 



■^1 N. H. Winchell : The geology of the north part of St. Louis County. Geol. and Nat. 

 Hist. Survey of Minn., Pinal Rept., vol. iv, 1899, pp. 255-257. 



'2H. L. Smyth: The Marquette iron-bearing district of Michigan. Monograph U. S. 

 Geol. Survey, vol. xxviii, 1897, p. 155. 



''^ 1. Morgan Clements : The Crystal Falls iron-hearing district of Michigan. Mono- 

 graph U. S. Geol. Survey, vol. xxxvi, 1899, pp. 112-124. 



