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



zone full of angular fragments of the blue phyllite, giving a 

 brecciated appearance. 



So far as could be observed, the topazes only occur in this 

 darker-colored earth, and this is in accord with the descriptions 

 given by Eschwege and Gorceix. This earth is evidently the 

 decomposition product of an eruptive rock which though 

 sheared is less perfectly laminated than the blue phyllite in 

 which it is inclosed and which was evidently much more irregu- 

 lar in its composition and structure. This last difference is 

 shown by a considerable variation, in streaks and patches, of the 

 coloration and by the occurrence of nodular masses from the 



size of a pea up to that of the fist, or larger, of white kaolin,, 

 of a dark chocolate-colored earth, or of granular quartz, or of 

 a mixture of these various elements. It is in these nodules 

 that the topaz occurs though in a somewhat irregular manner, 

 since a number of the smaller ones were washed without find- 

 ing a trace of the mineral. 



Of these nodular inclusions the most significant are those of 

 kaolin. The small group exposed on a carefully-scraped face 

 and shown of natural size in tig. 1 is very suggestive of an 

 original porphyritic structure somewhat modified by shearing. 

 The larger and more sheared group shown in fig. 2 exhibits a 

 gneissoid aspect suggestive of the shearing of a granitic or 

 syenitic nodule in a fine-grained rock as, for example in the 

 phonolitic types of the Serras of Tingua and of Caldas. The 

 larger nodules of white or chocolate-colored earth and of 

 quartz, in some but apparently not all of which the topaz 

 occurs, are too friable to permit of satisfactory sections, but 

 there can be little doubt that they are essentially of the same 

 character. These frequently show a scale-like crust and irregu- 

 lar intercalations from a few millimeters to one or two centi- 



