THE MINERALS IMPORTANT AS ORES, ETC. 25 



come the feldspars, and after these, when some residual silica re- 

 mains uncombined, it separates as quartz. * 



1.03.06. If we regard the igneous rocks as the source, the 

 metallic elements are thus to be ascribed to the first and second 

 series of crystallizations, while the elements of the gangue minerals 

 are derived from the last three. It is a doubtful point whether 

 the less common metals, such as copper, silver, and nickel, enter 

 into the composition of the dark silicates as bases,**replacing the iron, 

 alumina, lime, etc., or whether they are present in them purely 

 as inclusions of the first series. F. Sandberger 1 argues in 

 support of the first view, but his critics, notably A. W. Stelzner, 

 cast doubt upon his conclusions on the ground that his chemical 

 methods were indecisive. The case is briefly this : Sandberger, as 

 an advocate of views which will be subsequently outlined, sep- 

 arated the dark silicates of a great many rocks. By operating on 

 quantities of thirty grams he proved the presence in them of lead, 

 copper, tin, antimony, arsenic, nickel, cobalt, bismuth, and silver, 

 and considered these metals to act as bases. The weak point of the 

 demonstration consists in dissolving out from the powdered silicate 

 any possible inclusions. There seems to be no available solvent 

 which will take the inclusions and be without effect on the silicates. 

 This is the point attacked by the critics, and apparently with 

 reason. It is, however, important to have shown the presence .of 

 these metals, even though their exact relations be thus doubtful. 

 Quite recently in a series of " Notes on Chilean Ore Deposits " 

 (Tschermaks Min. und Petrog., Mitth. XIL, p. 195) Dr. Mori eke 

 mentions native gold in pearlstone (obsidian) from Guanaco, in 

 skeleton crystals in the glass, as inclusions in perfectly fresh 

 plagioclase and sanidine crystals, and in spherulites. The existence 

 of silver in quartz-porphyry has been demonstrated in this country 

 by J. S. Curtis, at Eureka, Nev.; 2 both the precious metals have 

 been shown by G. F. Becker to be in the diabase near the Com- 



1 The principal paper of Professor Sandberger is his " Untersuchungen 

 iiber Erzgange," 1882, abstracted in the Engineering and Mining Journal, 

 March 15, 22, and 29, 1884; but a long series of others might be cited in 

 which the investigations, notably at Pribram, Bohemia, are interpreted 

 as indicated above A. W. Stelzner, B. and H. Zeit. , xxxix. , No. 3. Zeitsch. 

 d.d.g. GeselL, xxxi. 644. " Die Lateral-secrrtions-Theorie, etc." Reprint 

 Freiberg, 1889. 



2 Monograph VII., U. S. Geol. Survey. 



