PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 327 



fornia, inasmuch as these arc pyritons ores, that is, ores formed by the mix- 

 ture of sulphur, either chemically or mechanically, with copper, iron, and 

 gold. The ores of California are native gold in veins of quartz. The first 

 specimen now presented is pyrites without any gxld in sight, although the 

 ore is of yeHow color, and might be mistaken iur the precious metal. The 

 next specimen is crystallized (piartz, having pyrites inclosed in the quartz, 

 showing that tiie gold was formed first, and was afterward enveloped by 

 quartz. Some of these crystals are very beautiful. Another specimen is 

 what is known as porous quartz, or quartz in which the pyrites have been 

 decomposed, and have disappeared, leaving behind the gold that was in com- 

 bination with it. Sometimes the g(jld in this variety of ore is visible to the 

 naked eye, and sometimes it exists as a thin film, staining the surface, 

 Avhich can only be seen with a magnifying glass. This variety, when in 

 sudicient abundance, is very profitable to work, because a part of the pro- 

 cess, that is, the decomposition of the pyrites, has already been performed 

 by nature. The third variety — and it may be remarked here that the twelve 

 specimens shown are all included in three varieties — is quartz ore having 

 pyrites and a portion of the wall rock of the vein attached. There were, 

 undoubted!}', many in the audience who had never seen a quartz vein. If 

 they have any curiosity in this direction, they can be gratified without 

 leaving Manhattan Islaiid. In the deep cut on First avenue, which is now 

 being graded at about Forty-second street, may be seen the gneiss rock 

 cut by a vein of quartz about a foot wide, descending at an angle of about 

 seventy degrees. The pure quartz is attached to the walls of gneiss on 

 either side; approaching the center are seen crystals of mica; still neare'r 

 the center are garnets, hornblenda and felspar. In some instances the 

 center is left open, in others it is closed, showing that the crystallization 

 proceeded from the tcalls toward the center, it being understood that this 

 deposit was made in a vent in the gneiss. To return to the Colorado speci- 

 mens, some were evidently formed in the same way. The veins of the Col- 

 orado resemble the veins of the Atlantic slope. The same kind of pyrites 

 veins are found in the gold bearing rocks of the Apalachian range. 



A few words may be added regarding the economy of mining or winning 

 gold. The question to be solved, is, how shall we separate the gold from 

 the iron, copper, sulphur and quartz 1 The sulphur may be driven off to a 

 certain extent by heat, that element being volatile. The quartz requires 

 crushing and grinding, after which it can be washed from the pyrites. The 

 gold is obtained from the quartz and all the other metals by means of 

 quicksilver, with which it amalgamates very readily. The quicksilver is 

 afterward distilled, and the gold is left as a solid. All this is easy to 

 describe, but, in practice, there stands your quartz, hard enough to cut 

 glass, and there your pyrites, harder than the quartz, and gold with the 

 whole in a variable quantity; s(jmetimes rich enough to repay you for your 

 great labor, alas! more often only in such appreciable quantities as to lure 

 the infatuated miner to greater expenditures of labor, time and money. 



The Law of Mariotte. 



Dr. W. Rowell read a paper on this law and its application in the steam 

 engine, from which the following important extracts are taken: 



