50 MINING INDFSTEY. 



united mass of quartz, and entirely distinct from the east ore-body, is tlie 

 northern prolongation of the west ore-sheet, terminated in depth with the 

 same oblique clay-plane, which plane joins, as far as can be seen, the west 

 selvage of the east vein. 



The main east vein conforms closely to the curved conchoidal lines of the 

 east country, and narrows from 200 to 60 feet, widening in depth again to 100. 

 The surfaces of this quartz are never smooth, but fractured masses are more 

 or less imbedded in the clay walls. This quartz is more and more shattered 

 in depth, becoming altogether sugary below 500 feet, like that of the lower 

 portion of the west vein. The red tint of the surface, given by percolations, 

 extends only to the depth of 300 feet in the east vein, but descends along the 

 clay-wall of the west vein to the 452-foot level. The east vein, in the deepest 

 workings of the Crown Point, Yellow Jacket, and Empire-Imperial, still con- 

 tinues with only slight signs of diminution. An increased mixture of clay and 

 small fragments of propylite is the only bad sign, and this, though usually a 

 fatal indication, has not yet proven so either here or in the Hale and Norcross. 

 At the south end of the 800-foot level of the Crown Point, the quartz begins 

 to be cut out transversely by propylite; but after a brief interruption of 300 

 feet it continues southward for a short distance into the Belcher ; see Atlas- 

 Plate 12. Lining these two veins of shattered quartz are. selvages of a plastic 

 gray clay, formed doubtless of decomposed propylite. It is of a tough con- 

 sistency, and when air is admitted by gallery or shaft it immediately begins 

 to swell and exert tremendous pressure, forcing itself through the interstices 

 of rocks, bending and breaking the most carefully laid timbers, and filling 

 mine- openings with extraordinary rapidity. The average thickness of the 

 east clay is two to three feet, but it expands in places to 30 and 40 feet. The 

 clays of the west vein are rarely over two feet, usually less; they are darker 

 in the region of the metamorphic rock, probably from the decomposition of 

 magnesian and iron minerals. Those clays which mark long fissure-lines 

 through the horse materials are of the same water-tight, tough nature. They 

 render mining drainage extremely uncertain; for reaching a given level is no 

 proof that all the region above will be drained, since the clays dam up within 

 the limits of conchoidal fractures a series of independent reservoirs which can 

 never be predicted. 



