330 X. H. WINCHELL SAPOXITE, THALITE, GREENALITE, GREENSTONE 



ated with sapoiiite oi' witli some of the species based on variations in 

 ''^serpentine/' 



T HALITE 



Thalite was discovered and named by D. T). Owen when he examined 

 the north shore of Lake Snperior.^ It was fonnd to l)e essentially a hy- 

 drated silicate of magnesia and alumina, formed along the shore of the 

 lake, from the alteration of ])asic igneous rock where the l)reaking waves 

 dashed over the rock. It occupies amygdaloidal and all shapeless cavi- 

 ties, some of the masses being several inches across, and sometimes it is 

 disseminated in hne granules through the mass of the decaying diabase. 

 Its color is usually dirty white or gray, but within the rock it frequently 

 is light green.^ By Dana this mineral was placed under saponite, to 

 which it has a chemical and physical likeness and a similarity of origin. 

 It appears to have as much right to recognition as an independent species 

 as several other species of a green color and soft and greasy feel, which 

 are of like origin and composition, derived from the decay of basic igne- 

 ous rocks, several of which have been embraced under the general term 

 serpentine. 



Greexalite 



Greenalite is a similar mineral having almost the same composition 

 and an identical origin. It is found to result from alteration of basaltic- 

 glass, or obsidian, an original constituent of the rocks of the Mesabi 

 iron range. '^ It was named by Leith in a report on the Mesabi district 

 in 1903," but had been discovered by J. E. Spurr several years before, 

 who regarded it as a non-potash form of glauconite. It is not only 

 sprinkled through the original rock, where considerable alteration has 

 taken place^ and where the iron and the silica also have become segre- 

 gated into individual masses, but it also serves as a general matrix, sur- 

 rounding the other secondary ingredients. The original basic rock in 

 this case was in the form of more or less rounded fragmental grains of 

 obsidian, and the greenalite retains quite frequently the subglobular 

 shapes of the fragmental grains. Leith has supposed the greenalite to 

 have been a chemical oceanic precipitate, in the form of a ferrous silicate 

 of magnesia and iron, from the waters of the cotemporary ocean, and to 



* Geological report on Iowa. Wisconsin, and Minnesota. 1S52. p. 600. 



■5 Final report. Geological Survey of Minnesota, vol. v. pp. 162. 168, 2.32. 2.'>S. 



* N. H. Winchell : "A diamond drill-core section of the Mesabi rocks." Proceedings of 

 the Lake Superior Mining Institute. 1910. 1911. 



' Monograph xliii, I'. S. Geological Survey. 



