256 QUICKSILVER DEPOSITS OF THE TAOIFIU SLOPE. 



aided by the ammonia. Tiie basalt is attacked by the acid waters and no 

 doubt by the sulphuric acid they contain. It is true that labradorite and 

 augite are but little acted upon by acids in laboratory experiments, but this 

 basalt has been exposed to the action of hot sulpliuric acid for hundreds of 

 years at least. Its resistance is also considerable, many kernels of fresh 

 rock remaining in the decomposed envelopes. 



Concentric decomposition. — It is clear from numcrous exposures that, after the 

 basalt solidified at the Sulphur Bank, it was divided bj' cracks, marking in 

 many cases a distinct though imperfect columnar structure. As usual, also, 

 there were cross-fissures in the vertical columns. These cracks formed the 

 passages by which ihe waters reached the surface and by which the acid 

 formed at the surface became diffused. The solid masses of basalt sepa- 

 rated by cracks from surrounding blocks were attacked from the outside 

 by the acid waters. As decomposition progressed successive shells were 

 formed, which grow more and more spherical as the centers are approached. 

 This has been attributed to "ball structure" in the rock, but it appears to 

 me unnecessary to assume any such predisposing cause, of which there is 

 no other evidence in the structure of this lava either macroscopically or 

 microscopically. It is shown in an earlier portion of this work (page 68) 

 that this conformation is the natural result of the action of a corrosive fluid 

 on a slightly porous, tolerably homogeneous material in blocks which 

 approximate to regular polyhedrons in form. The concentric shells which 

 are so well developed here are themselves the results of the decomposition 

 process and are not, in my opinion, pre-existing envelopes the presence of 

 which has controlled the course of decomposition. Decomposed basalts 

 showing this structure so strikingly do not occur, to my knowledge, else- 

 where in California, though such are found in other parts of the world. 

 An instance from Great Britain similar to tliat of Sulphur Bank is illus- 

 trated by Dr. Geikie.' 



Tlie ultimate residue, when the attack is complete, is almost pure silica. 

 The depth to which the basalt has been decomposed by the acid waters 

 varies in different exposures, and perhaps averages 20 feet. The limit is 

 usually very sharplv defined, and it may be considered certain that this 



' Text Book of Geology, 1st eil., Fig. 8G. 



