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



from acmite-trachytes and . phonolites mainly in the higher 

 proportion of silica, the analogy may be a closer one than at 

 first sight appears. 



Both Eschwege and Gorceix noted a linear arrangement of 

 the topaz localities which is also suggestive of occurrence along 

 one or more dikes. The older workings define quite clearly 

 two such lines lying several kilometers apart, but recent pros- 

 pecting in the soil cap shows that topaz occurs sporadically 

 over much of the intervening belt. 



Although the topaz is known to occur in other regions in 

 Brazil, very little definite information can be obtained regard- 

 ing it. Under the name of pingos de agua (water drops) rolled 

 white and blue pebbles were for a long time an article of com- 

 merce at the little town of Minas Novas on the Jequitinhonha 

 below Diamantina. This place was a center of trade in various 

 gem stones, other than the diamond (aquamarine, chrysoberyl, 

 tourmaline, spodumene, topaz and andalusite) coming from an 

 extensive region lying between the Jequitinhonha and Doce, 

 but for the most part, if not wholly, outside of the diamond 

 region proper, where it is to be noted the minerals mentioned 

 are extremely infrequent or wholly lacking in the miners resi- 

 dues that have been examined. Some years ago Dr. J. C. da 

 Costa Sena of the Ouro Preto Mining School made a trip in 

 this region and succeeded in finding most of these minerals 

 both in gravel deposits and in the rocks but makes no mention 

 of the topaz, from which it may be inferred that it does not 

 occur in association with the minerals above named. The 

 specimens from this region are often of considerable size, one 

 in the National Museum of Rio de Janeiro being but little 

 short of two kilograms in weight. 



Some years ago a gentleman at Serro between Ouro Preto 

 and Diamantina showed me some beautiful little doubly-ter- 

 minated crystals of white topaz that had apparently been 

 extracted from the original matrix and are presumed to be 

 from somewhere in the vicinity, but I failed to learn the exact 

 locality or conditions of finding. In the diamond region proper 

 the topaz is a great rarity, although Gorceix reports having 

 found it. Eschwege also reports it as occurring in a magnetite- 

 bearing pegmatitic rock on the island of Pescaria near the little 

 town of Sepitiba to the south of Rio de Janeiro. As there is 

 a massif of augite- and nepheline-syenite near by, a detailed 

 examination of this place may perhaps throw some light on 

 the hypothesis above presented, but owing to quarantine 

 restrictions it has not been possible to visit the place in time 

 for the present paper. 



Sao Paulo, Brazil, July 30th, 1900. 



