TOPAZ: OCCURRENCE IN BRAZIL 



335 



it is found as pebbles ranging in weight up to several ounces. Those of a dark shade are 

 the most valuable. 



The original situation of the blue and colourless topaz pebbles has not yet been 

 ascertained. The commonest rocks in the neighbourhood are granite and gneiss, and it 

 is probably from such that the stones have been derived. In many parts hereabouts there 

 is a surface layer, sometimes as much as 14 feet thick, of weathered material, consisting 

 mainly of quartz fragments, containing the same precious stones as are to be found in the 

 river beds. This weathered material has been derived from the underlying granite and 

 gneiss ; it seems very probable, therefore, that the precious stones contained in it had a 



Fig. 67. Occurrence of yellow topaz near Ouro Preto in Brazil. (Scale, 1 : 1,000,000) 



similar origin, having been formed, perhaps, in the quartz veins by which the underlying 

 rocks are penetrated. Small isolated grains of topaz and other prticious minerals are to be 

 found actually embedded as constituents in these rocks. The thick mantle of weathered 

 material, together with the precious stones it contains, is gradually carried away by the 

 action of running water, and dui'ing the transportation the stones assume the rounded form 

 characteristic of the pebbles of a river bed. The mode of occurrence of topaz in these 

 secondary deposits has a great general resemblance to that of diamond at Diamantina, 

 though at present diamond has not been met with in Minas Novas. 



The distribution and mode of occurrence of yellow topaz is quite different. Fig. 67 

 is a map of the district in which it occurs. The precious stone was here discovered about 

 the year 1760 in the neighbourhood of Ouro Preto, then known as Villa Rica, in the 

 south-west of Minas Geraes, of which State it is the capital. At that time yellow topaz 

 was in great demand, ar i the search for it was prosecuted with great vigour. It is most 

 abundant in a chain of hills extending for one and a half legoas (about six miles) in a 



