of the Gold-fields of California. 329 



in porphyritic pitchstone, in the leucite of the lava of Vesuvius, 

 and in the augite of some trappean rocks. In the case of quartz 

 from the auriferous veins of California, I am, however, inclined to 

 believe that they are due to crystallization only, since I have 

 never been enabled to discover globular vacuities in any of them, 

 and precisely similar markings may be observed on many quartz 

 crystals on examining their surfaces by the aid of an ordinary 

 lens. Should a fresh deposit of silica immediately take place on 

 the planes of a crystal thus marked, an appearance not unlike 

 glass-cavities would be the result ; and if the depressions on the 

 original crystal should become coated with clay, by deposition 

 from turbid water, before the formation of the succeeding stra- 

 tum of quartz, a certain degree of opacity, like that observed in 

 the quartz of many auriferous veinstones, would be caused. It 

 is also obvious that crystallizations of other minerals might take 

 place in such depressions ; and these, becoming covered by the 

 succeeding deposit of quartz, would have, to a great extent, the 

 appearance of having been formed from a slag or glass in the act 

 of cooling from a state of fusion. 



That crystalline quartz is frequently produced by successive 

 depositions we have abundant evidence from specimens in which 

 a crystal is seen to enclose others, each successively smaller 

 than that which immediately surrounds it. Clay and oxide of 

 iron are also in this way deposited on the surfaces of crystals 

 during the interval which elapses between the formation of 

 two distinct layers, as may be observed at the tin mines of 

 Schlackenwald and in other localities, where crystals are obtained 

 of which the various layers are so separated from each other by 

 a clayey deposit as to admit of being readily divided into a series 

 of cup-like envelopes, finally enclosing a central nucleus. 



Examination of Quartz taken from the Norambagua Mine. — 

 Specimens of quartz taken from a depth of 500 feet at the No- 

 rambagua mine, situated about four miles south of Grass Valley, 

 and having a specific gravity of 2'60, were selected for analysis 

 and afforded the following results : — 



Water lost at 212° F. ..... . 0-19 



Water lost by subsequent ignition . 0*39 



Silica 9629 



Alumina 1'42 



Iron 0'55 



Sulphur 0-57 



Arsenic traces 



Potash 0-36 



Soda traces 



Lime and magnesia traces 



99-77 



