542 



SCIENCE. 



[N. S. Vol. XVII. No. 431. 



the amount of water necessary for the forma- 

 tion of ore deposits as they are found in 

 nature. Professor Kemp has been a strong 

 advocate of the affirmative side of this ques- 

 tion. 



Professor J. P. Kemp, continuing the dis- 

 cussion, said : " In the establishment of types 

 of ore deposits we should seek certainties as 

 much as possible, and avoid cases which admit 

 of difference in interpretation. If we use 

 source and method of introduction of material 

 as a fundamental principle, we shall do well 

 in doubtful cases to fall back on points of 

 geological structure, since, regarding the facts 

 of the latter, there can seldom arise uncer- 

 tainty. As well-established types we have at 

 one extreme magmatic segregation from igne- 

 ous magmas ; at the other extreme, placers and 

 residual concentrates produced by water. 

 Starting now with contact deposits, produced 

 especially by the action of eruptives on lime- 

 stone and from pegmatites, which are assured 

 after-births of vulcanism, we may proceed 

 through the various types of ore bodies to an 

 extreme produced by raeteoric waters. Mr. 

 Weed has done a valued service in empha- 

 sizing the igneous causes, and surely no one 

 who appreciates the huge garnet zones and the 

 amount of silica contributed to them by the 

 eruptive, can fail to see in the eruptive itself 

 a rich source of quartz for veins. When we 

 appreciate further, as Mr. Lindgren has shown 

 for the gold deposits of North America, that 

 their formation was intense, relatively brief 

 and local, and that it followed the outbreak 

 of eruptions in each case, and that geological 

 periods and even eras passed without vein 

 formation, we must attribute great efficiency 

 to the eruptive rock. The dryness of deep 

 mines, now that it is realized, has greatly re- 

 stricted our old ideas of the amount of me- 

 teoric ground-water. The tendency, there- 

 fore, to emphasize igneous agents is well 

 justified, and is a distinct advance." 



Mr. T. A. Eickard referred to the want of 

 unanimity concerning the origin of ores, and 

 stated it as his belief that no scheme of classi- 

 fication would be generally adopted while au- 

 thoritative geologists remained so wide apart 

 in their conclusions. He pointed out that the 



trend of opinion had favored igneous or aque- 

 ous agencies at different periods in the history 

 of the subject, and that a gradual compromise 

 of views seemed to be the inevitable outcome. 



In Colorado it is a remarkable fact that the 

 profitable mines are distributed through every 

 geological terrain, from the Archean granite 

 to a Tertiary conglomerate, and mining is 

 going on in rocks belonging to all the prin- 

 cipal subdivisions of geological time and amid 

 a variety of petrographic environment which 

 includes nearly all of the principal sediment- 

 ary and crystalline rocks. In arriving at the 

 age of the country enclosing these lodes it 

 has frequently been difficult to consider the 

 sedimentary apart from the intrusive igneous 

 rock and it is not too much to say that there 

 is not a mining district, among the sixty-five 

 which he has tabulated, in which igneous 

 rocks do not occur in close association with 

 the ore deposits. 



Mr. P. L. Eansome, while not denying that 

 pneumatolysis might be an effective factor in 

 ore deposition, considered that the genetic 

 classifications recently presented to the so- 

 ciety carried this suggestive hypothesis fur- 

 ther than facts warrant. He illustrated some 

 of the objections to the extreme views of the 

 igneous school of ore-deposition by reference 

 to the occurrence of ores in the Mother Lode 

 district of California, the San Juan and Eico 

 districts in Colorado, and the Globe and Bis- 

 bee districts in Arizona. It was pointed out 

 that the important ore-bodies of these districts 

 were formed after the neighboring eruptive 

 rocks had solidified, and that pneumatolysis, 

 so far as known masses of igTieous rock were 

 concerned, was not directly active in ore- 

 genesis. His own experience led him to re- 

 gard the action of heated water, probably for 

 the most part of meteoric origin, as the most 

 generally effective agent in the formation of 

 the greater number of ore-bodies, as we know 

 them. 



Professor C. E. Van Hise stated that in 

 order to get a proper perspective for the ap- 

 preciation of differences of view, it would be 

 well first to give a summary of points of 

 agreement. Attention was called by the 

 speaker to the fact that, in his paper published 



