NEW IDRIA MINE. 305 



irreguliu- outlines and with ramifications extending into the walls alono- the 

 fissures to which they owe their origin ; but such deposits almost invariably 

 follow the fissure to a certain extent and present a large contact surface 

 with it. No better illustration of this usual relation could be given than 

 the Elvan Streak, with its accompanying bonanzas. Nothing of the kind 

 is apparent at the fault fissure. Where ore touches at at all, the surface 

 common to both is small. As a rule the fissure containing this clay seam 

 carries no ore whatever, while, had it been the channel of the ore-bearing 

 solutions, one would expect to find at least small quantities of cinnabar 

 nearly everywhere in it, as is actually the case in the Elvan Streak. It is 

 ver}' true that, in wide veins bounded by selvages of clay, the clay is often 

 barren; but such cases are not comparable with the ore-bearing ground of 

 the New Idria as a whole. The prevailing character and disposition of the 

 rock, together with the absence of a defined foot-wall, show that the stock- 

 works are not comparable to pockets in a vein, but to chambers adjacent 

 to a vein and impregnated from it. They are not included in a fissure, but 

 were charged with ore from a fissure. The stot-kworks stand to some fis- 

 sures in relations similar to those which subsist between the P^lvan Streak 

 and the irregular, contiguous ore bodies, and do not correspond to bunches 

 of cinnabar between the walls of the Elvan Streak, The term "pipe vein" 

 is used by von Cotta to describe some deposits analogous to these ; but this 

 expression, having been employed in different senses, is now objectionable. 

 A corresponding term is much needed to describe these adjuncts to fissures. 

 If the principal fissure of this mine be that at the clay wall, the clay, 

 in my opinion, fills the fissure. In that case the absence of cinnabar in 

 considerable quantity from this clay is \'ery strange, and would be so even 

 if the Elvan Streak and the New Hope did not exi.st. The fissures carry- 

 ing these two veins, however, were most assuredly in existence at the time 

 of ore deposition, showing that the physical character of the rock was not 

 such as to [)reclude deposition in fissures. This makes the absence of 

 ore from the clay wall inexplicable if it lies on the main fissure. On the 

 other hand, if the clay marks the position of a fault which took place later 

 than the deposition of ore, its own character and the relation which it bears 

 to the deposits are exactly such as would be expected. 



MON XIII 20 



