274 



SCIENCE. 



[N. S. Vol. XVII. No. 424. 



IV. Gas-aqueous (pneumato-hydato-genetic) 

 deposits. Igneous emanations 

 mingled with ground-waters. 



A. Filling deposits. 



B. Replacement deposits. 

 V. Meteoric waters. 



A. Underground. 



B. Surfieial. 



This classification is intended to group the 

 geological processes forming ore deposits in 

 such a raanner as to show genetic relations, 

 and to illustrate the subdivisions proposed by 

 actual examples, it being understood that in- 

 vestigators will differ as to which class a 

 particular deposit might be assigned. 



Major subdivisions are based upon mag- 

 matic segregations at one end, and cold 

 aqueous deposits at the other, with inter- 

 mediate groups due to the emanations from 

 igneous rock, the eruptive after-actions of 

 Vogt, to which the term pneumatolytic has 

 cormnonly been given; fumarolic when these 

 emanations issue at low temperature and pres- 

 sure; gas-aqueous in which the emanations 

 from igneous rocks, with their burden of 

 metals, mingle with ground-water; aqueous in 

 which meteoric waters alone are active, both 

 chemically and mechanically. 



The igneous deposits are divided into basic 

 and silicious, the former including the deposits 

 of iron, copper, etc., found at igneous borders 

 and as dikes, the latter the ore-bearing pegma- 

 tites with quartz veins as extreme examples. 

 Under igneous emanations or pneumatolytic 

 deposits are grouped contact metamorphic de- 

 posits shown by recent studies to be formed 

 under conditions which preclude the presence 

 of ordinary ground-waters or steam at a low 

 temperature and pressure. Pneumatolytic 

 veins, of which Cornwall tin veins are classic 

 examples, have long been recognized as due 

 to eruptive after-actions of this character. 

 Geikie, Fouque and other geologists have ob- 

 served the formations of metallic oxide in 

 clefts in lavas by fumaroles, hence this divi- 

 sion is introduced. 



Under gas-aqueous the larger number of 

 workable deposits occur, and it would be neces- 

 sary to present a long list of facts assembled 



to show their relations and the deductions 

 therefrom, to establish the necessity for this 

 subdivision; but if eruptive after-effects are 

 admitted to form contact metamorphic de- 

 posits, etc., the next group follows as a logical 

 consequence. 



Meteoric waters are admittedly the agents 

 that have by themselves formed large and im- 

 portant deposits of iron and copper, but this 

 agency is assigned to a less important place 

 than given it by recent writers. As a whole, 

 the classification differs very markedly from 

 any so far proposed, being the first to recognize 

 the facts established by Vogt, Lindgren, Kemp, 

 Spiirr and other advocates of the igneous 

 origin of ores. 



In the discussion of Mr. Weed's paper, Mr. 

 J. E. Spurr presented to the society a genetic 

 classification of ore deposits, upon which he 

 has been engaged for some years. He pointed 

 out a general similarity between this and the 

 classification proposed by Mr. Weed, especially 

 as regards the important place given to ore 

 deposits formed directly by igneous processes, 

 and the classes into which these deposits are 

 divided, the differences between the two 

 schemes being largely differences in relative 

 importance of the subdivisions and in detailed 

 grouping. Mr. Spurr expressed his full sym- 

 pathy with the theories of igneous origin for 

 ore deposits, and recalled his own advocacy of 

 these theories as early as 1894, when, . in de- 

 scribing the deposits of Mercur, Utah, a gase- 

 ous origin for one of the two types found 

 there was proposed, and a deposit in limestone 

 along a porphyry contact by waters occluded 

 from the porphyry for the other. Again in 

 1896 he argued that the gold quartz veins of 

 the Yukon district were the final silicious pro- 

 ducts of differentiation of a granitic magma. 



In his continuation of the discussion Mr. 

 Waldemar Lindgren admitted the desirability 

 of a genetic classification and believed that 

 the suggestions of Weed and Spurr should be 

 followed. Deposits formed by water above 

 the critical temperature by igneous emanations 

 and those formed by mingling of atmospheric 

 and igneous water are important divisions. 

 Fumaroles and solfataras are surface phe- 

 nomena and very different from deep-seated 



