392 H. W. TURNER 



under the microscope, a peculiar, coarsely granular structure, the grains being 

 partly bordered by crystallographic surfaces. This structure could have been 

 developed only by free crystallization in open spaces. It is scarcely necessary 

 to call attention, in addition, to the frequency of comb structure, etc., proving 

 also the same kind of origin. This does not necessarily mean that all large 

 bodies of quartz have been deposited in an open space, as large as the volume 

 of quartz now is. Repeated openings of the fissure have doubtless often taken 

 place. 



Lindgren's results as to the usually finely granular character 

 of the quartz deposited as a replacement are borne out in the 

 Diadem lode occurrence, as may be seen by an inspection of 

 Plate V on which are represented two photomicrographs, one of 

 a thin section of the red siliceous rock of the Diadem lode with- 

 out the analyzer, in which may be seen the outline of one of the 

 elliptical bodies previously referred to as being probably of 

 organic origin, and the other exactly the same view with the 

 analyzer, in which the finely granular character of the quartz is 

 shown. No careful microscopic examination of the quartz of 

 the large massive veins of the southern part of the lode has, so 

 far as I know, been made. Three thin sections from the crop- 

 pings at the Penon Blanco mine in Mariposa county (see Sonora 

 folio) show that considerable patches of the quartz have the 

 same optical orientation throughout, indicating large crystals 

 and consequently deposition in an open space. It appears, 

 therefore, likely that the huge quartz masses, some of them 

 twelve meters in width, were deposited in open fissures and are 

 not replacement deposits. The large size of the masses of quartz 

 having apparently the same optical orientation throughout and 

 the lack of banding may indicate merely quiet conditions during 

 deposition and lack of interruption of the process. 



To certain masses that form portions of the Mother lode a 

 somewhat different origin must be assigned. I refer to the oft- 

 described deposits composed of quartz, and calcium and magne- 

 sium carbonate, and mariposite. Lindgren considers 1 that these 

 large masses represent nothing but altered serpentine, and asserts 

 that abundant transitions may be found to prove this, as may 



'Bull. Geol. Soc. Am., Vol. VI, p. 235. 



