90 EDWARD STEIDTMANN 



under desert conditions. He applies this h5^othesis to the Algon- 

 kian of the Grand Canyon, the Torridonian sandstone of Scotland, 

 and the Sparagmite formation of northern Scandinavia. 



Matthew^ ascribes the sudden appearance of Cambrian life to 

 the obhteration of fossils by chemical changes and other causes, 

 partly to the absence or rarity of Benthos in the earlier faunas, and 

 partly to the fact that fossiliferous strata beneath the Cambrian are 

 arbitrarily attached to the base of the Cambrian. 



1911 



C. K. Leith and E. C. Harder^ state that Brazilian hematite ores 

 of Minas Geraes are found about 300 miles from the coast, in a 

 region constituted by a basement complex of crystalline schists, 

 upon which rests a sedimentary series consisting of slates, schists, 

 quartzites, ferruginous quartzites or itabirites with interbedded 

 hematite ores, gneiss, ferruginous carbonate rocks, carbonate rocks, 

 and amphibolites. The sediments have been severely folded and 

 altered by granitic intrusives. Near the bottom quartzite pre- 

 dominates. Argillaceous sediments predominate near the top and 

 the iron ore beds are most abundant near the middle of the sedi- 

 mentary series. 



The most abundant ores of the district are thin-bedded, fine- 

 grained hematites, of excellent grade. Thick-bedded, massive, 

 nearly pure hematites rank next in importance. 



A very large tonnage of fragmental ore consisting of residual 

 hematite and more or less transported debris resulting from the 

 breakdown of the ore deposits is in sight. These ores are cemented 

 with sands, clays, and limonite and therefore are lower in grade than 

 bedded deposits. A negligible quantity of ore is known to have 

 developed from the leaching of ferruginous carbonates and itabirite. 



The Lake Superior ores have resulted from primary deposition 

 and secondary concentration. In no known case has primary 

 deposition alone developed ore. All the ores are related to structu- 



I G. F. Matthew, "The Sudden Appearance of the Cambrian Fauna," Compie 

 Rendu, Congres Geologique International, 1910, pp. 547-551, 



=> C. K. Leith and E. C. Harder, "Hematite Ores of Brazil and a Comparison with 

 Hematite Ores of Lake Superior," Econ. GeoL, VI (1911), 670-86. 



