DISTRIBUTION^UNITED STATES 615 



proving the individuality of the masses before solidification, as shown by 

 Dathe and Dalmer, and hence they can not be due to weathering. He 

 compares them with the block lavas of Santorin and the aa la^'as of 

 Hawaii and attributes the structure to a 



"slow forward movement and contemporaneous breaking up of the viscous 

 lava. . . . This is the shape which viscous material would naturally tend 

 to take when subjected to the rolling action attendant upon the onward motion 

 of the stream of which they form an outer portion, or in certain cases the 

 entire thickness." 



The rounding of the masses is thought to have been caused chiefly by 

 rolling, but contraction due to cooling was regarded as a contributory 

 cause, and later compression has broken ofl the jagged portions and 

 partly filled the interstices. 



Pillow structure is found almost universally in the Ely greenstones of 

 the Vermilion district of Minnesota (plate 17, figure 2), with essentially 

 the same characteristics as in the Crystal Falls district. Clements,^* 

 who describes the features of this structure in great detail, calls attention 

 to the fact that in some places the amygdules are concentrated chiefly on 

 one side of the pillows (the upper side), and that perfect transitions are 

 found between the ellipsoidal and the nonellipsoidal basalt, both of essen- 

 tially the same grain. The author considers these rocks to have had the 

 same history as the similar ones at Crystal Falls, and hence that the 

 shape of the ellipsoids was determined to a great extent by the rolling 

 over and over of the blocks of aa lava, while still retaining some plas- 

 ticity. At the same time they were contracting through cooling, and 

 possibly subsequent pressure may have molded them to a certain extent. 



Where they have been subjected to great pressure the ellipsoids have 

 been mashed into disk-shaped bodies, and in extreme cases have been 

 drawn out into bands, the material of the ellipsoids alternating with the 

 thinner bands from the matrix. 



Clements calls attention to the wide range of this structure, both geo- 

 graphically and geologically, in the various districts of the Lake Superior 

 region. 



"Thus, for example, it has been described from the Marquette and Crystal 

 Falls districts of Michigan, and one can state with a fair degree of assurance 

 from the occurrence of large quantities of greenstones in the Penokee-Gogebic 

 of Michigan and Wisconsin that it also occurs there, although it has not been 

 described from that district. It has also been observed by the writer in a 

 number of places in the Menominee district of Michigan and in the Mesabi 



'* J. Morgan Clements : The Vermilion iron-bearing district of Minnesota. Monograph 

 XJ. S. Geol. Survey, vol. xlv, 1903, pp. 144-152. 



