616 J. v. LEAVIS ORIGIK OP PILLOW LAVAS 



district of Minnesota. Lawson describes it in the rocks of the Lake of the 

 Woods region. The same structure has been described from the Michipicoten 

 iron-bearing district on the east side of Lake Superior by A. B. Willmott, and 

 Dr. S. Weidman, of the Wisconsin Geological and Natural History Survey, 

 states that it occurs in greenstones of supposed Huronian age in the vicinity 

 of Wausau, Wis. It has been observed in the Archean greenstones of Lake 

 Nipigon, in Ontario, Canada. As a result of field studies in the Keweenawan 

 volcanics of the north shore of Lake Superior in 1900, the writer knows that 

 it occurs also in them, . . . The rocks in which it occurs range from the 

 Archean of the Vermilion district of Minnesota and the adjacent Canadian 

 districts and the Marquette district of Michigan to the Keweenawan. It oc- 

 curs within the greatest superficial area of the Archean." 



According to Leitli,'^ tlie structure of the basalts aud inetabasalts of 

 the Mesabi district is identical with that of the various other districts to 

 which reference has already been made. , 



"The ellipsoids themselves consist of basalt and vary in diameter from a 

 few inches to one or two feet. They are separated by narrow bands of some- 

 what lighter or darker basalt. The ellipsoidal .structure is one supposed to 

 have been induced in the rock when it first cooled from an extensive magma, 

 perhaps subaqueous." 



Tan Hise and Leith,'*^ in summing up and revising the geology of the 

 Lake Superior region, describe again the characteristic ellipsoidal struc- 

 ture of many of the greenstones, which exhibit every gradation from 

 typical pillow lavas to schists that show a banding, owing to the differ- 

 ence in color between the deformed ellipsoids and their matrix. All of 

 the iron-bearing formations, believed to be aqueous sediments, are "asso- 

 ciated with basalts having conspicuous ellipsoidal structure which can be 

 best explained as developed by flowing out under water." These are 

 strongly contrasted in this respect with the basic lavas of the Keweena- 

 wan series. 



After reviewing some of the studies that have led to diverse views on 

 the question of origin, the authors conclude that 



"the evidence seems to be that the ellipsoidal structure is both subaqueous 

 and subaerial in its development, that it is produced by the rolling of the 

 blocks developed during the flow of the lava as a result of cooling, and that 

 its development is therefore determined by the speed of flow and the rate of 

 cooling, which in turn may be affected by entrance into water." 



The interbedding of the basaltic flows with subaqueous sediments in the 



■^ C. K. Leith : The Mesabi iron-bearing district of Minnesota. Monograph TJ. S. Geol. 

 Survey, vol. xliii, 1903. p. 65. 



'« C. R. Van Hise and C. K. Leith : The geology of the Lake Superior region. Mono- 

 graph r. S. Geol. Survey, vol. lii, 1911, pp. 120, 151, 502, 510-512. 



