462 



SCIENCE 



[N. S. Vol. XXXIII. No. 847 



of Mr. Brock, the vice-presidential address on 

 "Northern Canada" was not read on Wednesday- 

 afternoon, but read by title only. 



file Weathering and Enrichment of Pyritic Gold 



Ores: W. H. Emmons. 



Any theory of leachmg or enrichment at depth 

 of auriferous deposits involves the solution, trans- 

 portation and precipitation of gold in cold and 

 moderately dilute solutions. The analysis of 

 underground waters shows that sulphuric acid and 

 sodium chloride are present in practically all 

 waters from pyritic gold mines. Gold is dissolved 

 by nascent chlorine, which in acid solutions is set 

 free by nitric, manganitic, ferric or cupric com- 

 pounds. Since nitrates are seldom shown in the 

 analyses, they are believed to be of little or no 

 importance in this connection; ferric and cupric 

 compounds release nascent chlorine only in hot or 

 in concentrated solutions. As shown by Brokaw's 

 experiments appreciable gold is dissolved in four- 

 teen days in cold solutions containing but 1,418 

 parts per million of chlorine, when manganese is 

 present. In a similar solution without manganese 

 no gold is dissolved. As shown by McCoughy a 

 very smaU amoimt of ferrous salt will precipitate 

 gold dissolved in chlorine solution. Consequently 

 the auriferous solution could not travel far 

 through deposits where pyrite is oxidizing to fer- 

 rous sulphate. But manganitic compounds such 

 as pyrolusite very quickly oxidize ferrous salt to 

 ferric salt. Thus manganitic compounds not only 

 release the nascent chlorine which dissolves gold 

 but they also inhibit the precipitation of gold by 

 ferrous salt, thus permitting the gold to travel 

 farther in solution. 



But these laws apply only to acid solutions. 

 At greater depths where acid is used up to form 

 neutral or basic salts by reactions with the wall 

 rock, the system breaks down and gold together 

 with manganese oxide is precipitated. Indicating 

 these reactions certain manganiferous gold de- 

 posits are found in depth to consist of pyritic 

 gold ore cut by fractures which are filled with 

 rich veinlets of auriferous powdery manganese 

 oxide (probably manganite, in the main). 



It follows from these premises that mangan- 

 iferous gold deposits are less likely to yield gold 

 placers than non-manganiferous deposits; that 

 outcrops of manganiferous lodes are more likely 

 to be leached of gold near the surface; that non- 

 manganiferous lodes are less likely to contain 

 secondary gold bonanzas. It is believed that 



these relations are indicated by the pyritic gold 

 deposits of the United States. 



The Genesis of Certain Greensands of Minnesota: 



N. H. WiNCHELL. 



The greensands discussed were: (a) that which 

 occurs in the valley of the Blue Earth River, dis- 

 covered in 1700 by Le Sueur and mined by him in 

 the belief that it was an ore of copper, and (fc) 

 that which occurs on the Mesabi iron range, caUed 

 glauconite and greenalite, and regarded by some 

 as the source of the iron ore of that range and 

 of other Lake Superior iron ranges. 



The former was shown to be so closely asso- 

 ciated with the residual products, such as kaolin, 

 that it must be considered to result from the 

 decay of the Upper Cambrian strata with which 

 it is found. The chemical and physical characters 

 were given, samples were shown, and finally, pho- 

 tographs of the bluffs showing the deep disin- 

 tegration of the strata along the Omaha Eailroad. 

 Kidney iron ore (with the shape of limestone 

 pebbles, which have been a standing puzzle to 

 Minnesota geologists) was another product of 

 this same disintegration, which was in some way 

 dependent on the presence of the Cretaceous 

 ocean, which covered the locality. 



The greensand of the iron ore ranges of Min- 

 nesota has been reinvestigated by means of thin 

 sections of a drill core furnished by Mr. E. J. 

 Longyear, of Hibbing. This core was representa- 

 tive of a drill that went through the Mesabi rocks, 

 to a depth of 2,049 feet. One hundred and six 

 thin sections were examined. Throughout the 

 core, beginning at the top of the black slate and 

 continuing to near the horizon of the iron ore, the 

 rock is characterized by the predominance of an 

 isotropic substance in grains of all shapes, evi- 

 dently sedimentary, though seldom in distinctly 

 detrital or rounded forms. The supply of ma- 

 terial was so rapid that there was not enough 

 friction to round the grains thoroughly. Mingled 

 with these grains are angular grains of quartz 

 and other minerals. The isotropic grains resem- 

 bling glass, are seen to contain bubbles and glob- 

 ular incipient minerals. They are sometimes 

 slightly greenish, and sometimes brownish, but 

 have a high index of refraction. They are some- 

 times partly and even wholly opaque, and black, 

 and when opaque they are variable in form. 

 These obvious primary microscopic characters in- 

 dicate that this isotrope is of volcanic glass, and 

 the secondary microscopic characters agree with 



