Derby — Occurrence of Topaz near Ouro Preto, Brazil. 27 



these bluffs gives a small residue of rolled zircons, most 

 abundant in the more quartzose one, and there can be no doubt 

 that it is a metamorphosed clastic although from its essentially 

 micaceous and ferruginous character, indicating a high per- 

 centage in alkalies and iron oxide, it may be suspected that it 

 was not originally a perfectly normal sedimentary clay. On 

 the slope of the col itself and only a few meters away from the 

 mine on the line of strike there is a small exposure of a soft 

 bluish black slate-pencil phyllite, which, in appearance, only 

 differs from the rock of the bluffs on either side in its finer 

 grain, and which is in all respects identical with the one of 

 which an analysis was given in the September number of this 

 Journal from a smaller topaz working in the immediate 

 vicinity. This is a sericite schist free from quartz but heavily 

 charged with iron, and in the paper above cited its low silica 

 and high alkali coutents were given as an argument for con- 

 sidering it as, possibly, a sheared eruptive. The absence of 

 rolled zircons in its heavy residue (both the rock at Caxambu 

 and the one analyzed) may be cited as a confirmation, though 

 not an absolutely conclusive one, of this argument. 



The mine itself, which is simply a small excavation in earthy 

 material which when soaked with water has much the appear- 

 ance and consistence of soft soap, exposes a zone some ten or 

 dozen meters wide of bluish and yellowish clays which still 

 distinctly show the nearly vertical lamination of the rocks 

 from which they are derived. The predominant color is bluish 

 but with streaks and patches of yellow which in part represent 

 a more advanced stage of decay of the bluish material, in part, 

 as will be seen farther on, a material of somewhat different 

 structure and composition. The blue clay could be very satis- 

 factorily traced into the slate-pencil phyllite above mentioned 

 and on the other hand into a light yellow micaceous clay, or 

 rather earth, that shows a change of color owing to the more 

 complete hydration of the iron oxide. In the midst of this 

 earthy material that, while still retaining its original form and 

 structure, falls to pieces on the slightest movement, are bands 

 from a few centimeters up to a meter or more in width of a 

 different colored and more plastic earth, carrying oxide of man- 

 ganese as well as of iron that gives it a darker yellow color 

 passing to brown or black. The contacts between these two 

 kinds of earth are perfectly sharp, like an eruptive contact, and 

 are sometimes accentuated by a concentration of iron and man- 

 ganese oxides, forming a black sahlband of about a centimeter's 

 width that merges gradually into the general mass of the 

 brownish or yellowish earth. In one case a band was observed 

 having such a black sahlband on one side and on the other a 



