1064 A TREATISE ON METAMOEPHISM. 



Under present conditions mines may be dry even when the openings 

 have not been closed by cementation. For example, very recently in New 

 Mexico and Arizona there have been important climatic changes. It is 

 well known that these districts were humid in the Quaternary period, during 

 at least two different epochs. During these times the level of underground 

 water was probably as near the surface as it is in the humid districts of the 

 west at the present time. Therefore in these districts in late geological 

 time there was a vigorous underground circulation at those places where 

 cementation was not complete. 



The dryness of mines which at the time of the vein filling had a vig- 

 orous circulation may be due either to cessation of circulation by cementa- 

 tion or to cessation of circulation in arid regions in consequence of chang-e 

 from humid to arid conditions. 



It seems to me that where the gangue minerals are those known to be 

 deposited by aqueous solutions, and the relations of the vein fillings are 

 such as to show that they were deposited contemporaneously with the gen- 

 eral cementation material, these facts are decisively in favor of action of 

 ■aqueous solutions notwithstanding absence of vigorous circulation at the 

 present time. Those who arg*ue that the ores were not deposited by aque- 

 ous solutions because there is not now a vigorous aqueous circulation have 

 equally good or better ground for stating that the ores were not deposited 

 by gaseous solutions; for certainly in regions of ore deposits where there 

 is not now a vigorous aqueous circulation there is still more notably a 

 deficiency in gaseous circulation. Bj a strange inconsistency some of the 

 men who have held that the main class of ores is not deposited by aqueous 

 solutions, and have used as an argument- for this belief the absence of a 

 present vigorous circulation, have jumped to the conclusion that such ores 

 were therefore deposited by gaseous solutions, not taking the trouble to 

 ask whether the same argument applied in this case. 



In this connection it is to be recalled that in those cases where actual 

 vein filling is now known to be going on, as at Steamboat Springs, Sulphur 

 Bank, Boulder Hot Springs, and various other localities, there is now a 

 vigorous aqueous circulation. Also in some cases of very recent vein 

 formation, as at the Comstock lode and Cripple Creek, where the openings 

 were not fully closed by cementation, an enormous amount of water is 

 handled in mining. The Portland mine at Cripple Creek, Colo., from 1898 



