164 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



for a person acquainted with the region to distinguish a small sample of river-deposit from 

 one of valley-deposit. 



The " cascalho " of the valley-deposits rests, as a rule, not directly upon the solid rock, 

 but upon a variously-colom-ed layer of fine sand mixed with clay ; this is not of any gi-eat 

 thickness, and is called " barro." It also contains diamonds, and passes gradually, with no 

 sharp line of demarcation, in bo the "cascalho" above. The "barro," however, is always 

 distinctly bedded, while the true " cascalho," whether on the sides or the bottom of the 

 valley, shows no signs of bedding. It is often, but not invariably, covered by a layer of red 

 muddy earth. 



The " cascalho " of the sides of the valley is usually less rich in diamonds than is that 

 found at the bottom ; the stones it does contain, however, are less worn and rounded, and 

 relatively larger than those found in the present river-bed. 



Plateau-deposits are found at numerous spots on the hills near Diamantina, and in 

 other diamantiferous districts of Minas Geraes. A rich yield of diamonds was obtained 

 from many of these in former times, but at the present day a few only are worked, and these 

 are of less importance than are the river-deposits. 



On the hills near Curralinho (Fig. 35), between the Rio Jequetinhonha and the town 

 of Diamantina, lying about due east of the latter, are the rich mines Bom Successo and Boa 

 Vista. On the plateau south-west of Diamantina, and between the basins of the Rio 

 Pinheiro and the Rio Pardo Pequena, are the mines La Sopa and Guinda, which are now 

 being worked, there being here two diamond-bearing beds of different ages one above the 

 other. 



Further on in the same direction, and about twelve miles west of Diamantina, in the 

 district where the Rio Caethe Mirim and the Rio Pinheiro take their origin, are the specially 

 noteworthy deposits of Sao Joao da Chapada, which wiU be described below. A little to 

 the south of this place, in the neighbourhood of the source of the Rio Ouro Fino, are the 

 diggings of La Chapada, which were formerly very rich, and are now only partially 

 exhausted. 



As regards the character of the plateau-deposits, the material of which they consist is 

 very similar to that of the river-deposits, differing from it, however, in the presence of a 

 larger proportion of the heavier of those minerals usually associated with diamond. This is 

 to be expected, seeing that the lighter materials would be the more easily carried away by 

 running water, and the heavier minerals more liable to be left behind. Thus we find the 

 oxides of titanium and of iron present in great abundance, though even here the amount is 

 exceeded by that of the different varieties of quartz. The material of these plateau-deposits 

 is known as gurgulho, it occurs usually in horizontal beds, and is built up of coarse blocks of 

 the surrounding rocks, with a more or less red clayey earth. In this material the diamond 

 and its associated minerals are embedded; so indiscriminately, however, is everything 

 coloured by the red clayey earth that it is impossible to distinguish one mineral from 

 another until the material has been washed. In some deposits the earthy material has been 

 removed by a natural process of washing, and here the diamonds and their associated 

 minerals are from the first distinctly seen. The rock and mineral fragments in the 

 " gurgulho " are very slightly, if at all, rounded, and the diamonds themselves still 

 preserve their perfectly sharp edges and corners, and the original natural characters of 

 their faces. 



The proportion of diamonds and their associated minerals present in a given weight of 

 the material is rather smaller in " gurgulho " than in other deposits, but the average size of 

 the stones is greater. The distribution of the diamonds is sometimes very irregular, large 



