winchell's studies of basic ignp:ous rocks o25 



obsidian sand, was elieinicaJly ciiaiiged, the jnost of the non-soluble in- 

 gredients being segregated into favorable locations, the soluble being 

 taken into the oceanic waters and removed. Silica penetrated the un- 

 crystallized or rhyolitic lavas and obsidian sand, giving rise not only to 

 the taconite and jaspilyte masses, but also to the secondary quartzose 

 grains, retaining their original shapes in the same manner that the silici- 

 fied forms of trees and other vegetation are seen to be preserved in the 

 alkaline, often volcanic, sediments of the Tertiary in the central western 

 states. From the ferriferous solution of the near-by waters of the ocean, 

 iron carbonate seems to have been j^recipitated in considerable amounts, 

 such deposit becoming a constituent part of the cotemporary sedimentary 

 strata, and in all probability, in favorable situations, ferric oxide was 

 also deposited, as well as silica, these three forming alternating ingre- 

 dients in the strata as now observed. The existence of iron-carbimate, 

 however, is quite subordinate in Minnesota, so far as discovered, and 

 this statement here depends mostly on reports that have been made hy 

 others as to its abundance on the south side of Lake Superior. It is 

 almost lacking in the Vermilion and Cuyuna ranges, and in the produc- 

 tive parts of the Mesabi range it is replaced by carbonate of lime; on the 

 eastern extension of the Mesabi, as at Gunfiint Lake, it is more common. 



The greensand (greenalite), which has attracted so much attention, 

 was shown, in 1900, to graduate in size into larger and larger masses, 

 and in character into the taconitic '^'horses" and other residual forms 

 of the primary lava. It never constitutes distinct strata. 



In no case and in no particular has the writer abated from this view. 

 Indeed within a few years past he has reinvestigated the subject, both in 

 the field and by the aid of microscopic thin sections, and has found 

 overwhelming additional evidence to confirm its correctness.^"* He has 

 (m several occasions since* 15)00 r(>i)eat(Ml Ihis theory in Ihe American 

 Geologist and elsewhere. He has later shown that greenalite itself is not 

 an oceanic precipitate, but a product of alteration of the original lava.^^ 



In a recent publication by the United States Geological Survey^** it is 

 shown conclusively that the greensand could not have been derixcd bv 

 weathering and drainage from ])reexisting l)asic greenstones of the ic- 

 gi(m. It had previously been shown Ijy Leith that it is not of organ ie 

 origin, as supposed by Spurr. It is also plain that as a silicate of iron 

 it could not l:ave existed in such large quantities locally in ihc wntei- of 

 the ocean without an iinmediate local cause. It became neeessarv, therc- 



" Proceedings of the Lake Superior Miuiiifx Institute, IKON. l'.U)!». I'.no. inil. 

 '•"Proceedings of tlie Lal<e Superior Mining Institute. 1J)1(». 



'« Van Ilise and Leitli : "The geology of tlie Lal<e Superior region." inonograi)ii 111 

 1911. 



