GREAT MOTHER LODE OF CALIFORlSriA. 65 



pyrites, mispickel, marcasite, chalcopyrite, galena, &c., or "by- 

 oxidised products resulting from tlieir decomposition. They are 

 mostly found to be somfwliat auriferous wherever they have 

 been tested, and some of the mines add considerably to their 

 returns by roasting their concentrates and milling them, the free 

 gold having been first taken out of the stamped ore (pulp) by 

 the aid of mercury.* 



Prof. Whitney goes on to say : — " This immense mass is not 

 by any means proved to be a fissure vein, or even an exclusively 

 segregated one. It will require much more study than it has yet 

 received before its real character can be stated with confidence. 

 To the writer it seems from present evidence most likely that it 

 is the result of metamorphic action on a belt of rock of peculiar 

 comjDosition, and perhaps originally largely dolomitic in charac- 

 ter." (Whitney, p. 332.) 



As to the age of this vein, we can only say with certainty 

 that it is later than the elevation of the Sierra Nevada. This 

 appears from the fact that the quartz itself is rarely faulted or 

 disturbed to any considerable extent, as it must have been had 

 it existed before the uptilting of its enclosing rocks. Never- 

 theless, the powerful lateral compression to which the belt has 

 been subjected by that upheaval may, on being relaxed, have 

 given rise to the cavities into which the quartz gradually became 

 segregated, together with the gold and its accompanying pyri- 

 tous matter. 



The accompanying sketch map, taken from a survey by Mr. 

 E,. H. Stretch of San Francisco, indicates the positions of the 

 principal mines which are now being worked on this lode, or 

 which have been profitably worked in recent times. Many of 

 them are shallow, but some few have been carried down to depths 

 of 1000 feet or more without any signs of the gold giving out. 



As to the richness of the vein, of course this varies exces- 

 sively, as is always the case in mineral veins. In some localities 

 only traces of gold are present, in others many tons of work 

 have been extracted yielding 100 or more ounces to the ton. 

 More usually, however, the milling ore will vary from 3 dwts. up 



* In a few instances other ores are, or have been, worked in veins situated 

 within a few miles of the Mother Lode — thus telluride of silver ores occur at 

 Modesta Mine in Mariposa, and copper ores at Copperopolis in Calaveras. 



