DEPOSITS OF THE DEEP CIRCULATION. 1133 



evidence of the ascending waters. One of the common causes for the 

 cessation of a vigorous underground circulation is the nearly complete 

 cementation of the openings by the circulation. When the openings become 

 subcapillary the circulation necessarily ceases. 



Where there exist underground streams with steady flow the channels 

 which they occupy are usually clean and fresh. They are lined with beau- 

 tiful clear crystals without a stain. Along the channels there may be open- 

 ings or even caves lined with crystals of various sizes, some of them reaching 

 a foot or more in diameter. Such vugs, openings, and caves are superbly 

 illustrated in the Joplin district of Missouri, where, in consequence of the 

 lowering of the level of ground water by mining, such openings may be 

 visited. When first freed from water the crystals lining the cavities are as 

 untarnished as polished plate glass. The same thing may be seen at the 

 Enterprise mine at Platteville, Wis., in the Upper Mississippi Valley. At 

 this locality, as well as many in Missouri, the ores are clearly associated 

 with the trunk channels of circulation. The miners in their operations 

 follow the channels as guides, working- against the streams. At places 

 along' a channel the water is concentrated into a single stream, and at such 

 places there is likely to be comparatively narrow belts of very rich material 

 along the walls. In other places the stream is subdivided among many 

 small openings, and at such places the ore is dispersed. 



Probably this criterion of ascending waters and freshness of the depos- 

 ited material furnishes the best single line of evidence in favor of a deep 

 circulation. But one must not conclude that the deposits of the ascending- 

 waters extend, unaffected by descending' waters, to the very surface. 

 For instance, while the present mining in the Joplin lead and zinc district 

 of Missouri is in the belt in which descending waters have produced com- 

 paratively little effect, i. e., from 30 to 60 meters deep, at the surface and 

 for a few meters below the surface descending and ascending waters mingled. 

 Together they moved off laterally, and in consequence of their joint action 

 the oxidized products and the galena deposits mentioned on pages 1444- 

 1446 were produced. In this particular area the distance to which the 

 descending water produced an effect is very small, but such is not univer- 

 sally the case. Where the vertical elements are greater and the relief is 

 steeper, the descending waters working' against the ascending currents may 

 produce an effect to a very considerable depth. 



