HEAT rriENOMENA. 239 



gressive decomposition towards the outside. Large masses of fresh rock 

 also occur in a similar way, as has been described in the discussion of pro- 

 pylite. It is clear from these occurrences that had the decomposing action 

 been prolonged sufficiently, no undecomposed rock would have remained. 

 Under ground the decomposition is more universal, if one may judge from 

 the Sutro Tunnel. From Shaft II. to the Lode no fresh rock is exposed by 

 the tunnel, except the small mass of diabase close to the hanging wall which 

 has been referred to. This marked difference between the superficial and 

 subterranean rocks should be considered in connection with one of the 

 deductions made in discussing the structural results of faulting — viz., that 

 the country has undergone but little erosion since the deposition of the ore. 

 Indeed, it may be regarded as independent evidence tending to the same 

 conclusion. 



Rocks involved. — The three rocks which occur in the belt of highly decom- 

 posed east country are diabase, hornblende-andesite, and augite-andesite. 

 The andesites are found extensively in other portions of the Distkict, 

 where, however, they are decomposed to but a trifling extent. There is 

 no reason known to me to suppose that the decomposed andesites are of 

 different eruptions from the fresh occurrences ; on the contrary, the decom- 

 position dies out gradually in continuous areas. Neither is there any 

 evidence that the fresh and the altered masses are of a different composition. 



Evidence of an external cause. — Had the resolutlou of tlic complex rock min- 

 erals into simple compounds been spontaneous, the nodules of rock described 

 could not have formed, for the action must have been nearly uniform 

 throughout. Neither could they have been formed if the presence of moist- 

 ure had been sufficient to induce decomposition, for all rocks, except perhaps 

 obsidian, are permeable by water. Solutions of carbonic acid, hydrosul- 

 phuric acid or the like, on the other hand, if brought in contact with com- 

 pact masses of material susceptible to their action, would grow weaker as 

 they penetrated towards the centers of blocks, and would bring about just 

 such results as those refen-ed to. 



Kvidencethat the solutions ascended. — If surface waters had produccd the decomjio- 

 sition, the andesites at the surface throughout the District would have suf- 

 fered nearly uniformly, and the amount of decomposition must have decreased 



