492 GEOLOGY AND MINING INDUSTRY OF LEADVILLB. 



The influence of cross-cutting sheets of porphyry in producing a con- 

 centration of rich ore by causing a stagnation of the ore currents is shown 

 by the distribution of the ore shoots. Thus the northeastern body in the Cli- 

 max, Dunkin, and Matchless ground lies under the lee of the cross-cutting 

 sheet of White Porphyry; the main ore shoot in the Amie, Pittsburgh, Little 

 Chief, and Chrysolite lies in a similar position relatively to the Gray Por- 

 phyry dike; and the southern body in the last three claims lies just north of 

 the lower cross-cutting sheet of Gray Porphyry. 



The greater secondary alteration on this hill is readily accounted for 

 by the fact that it is everywhere covered by a great thickness of Wash. This 

 Wash, which is a loosely aggregated and permeable bowlder clay, acts like 

 a wet sponge. It is constantly full of water at its contact with the rock 

 surface on which it rests, which water is doubtless charged with air and de- 

 composed vegetable matter, and thus acts more vigorously upon the rocks 

 than would water flowing freely over the actual surface of the ground 

 or that which percolates in minute channels through the solid rocks beneath 

 the surface. This is shown by the fact that the upper sheet of White Por- 

 phyry, which lies immediately beneath the Wash, is generally reduced to a 

 plastic mass, in which all trace of the original structure of the rocks is lost, 

 while the lower sheet of the same rock is still a hard, compact rock, forming 

 what the miners call block porphyry. 



