162 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



considered unimportant. Opinions on this matter naturally vary, and are to a certain 

 extent arbitrary, but it may be taken as a safe rule that the presence of those minerals, 

 which are most constantly associated with diamond, is an indication which must not be 

 disregarded. 



We now pass to a more detailed consideration of the three classes of diamond-bearing 

 deposits, namely those of the rivers, valleys, and plateaux, as they occur in the district of 

 Diamantina and elsewhere in Minas Geraes. 



The river -deposits are the richest of the three ; they are found in the valleys below 

 the existing high-water level, and at the present time are the only deposits of importance, 

 not only in this district but also in the whole of Brazil, and this in spite of the fact that 

 the average size of the stones so found is smaller than that of the diamonds of other 

 deposits. In connection with the question of size, it is remarkable that stones found in the 

 lower courses of a river are smaller than those in the upper part, and that eventually a point 

 is reached in the river course at which the diamonds disappear altogether. This is strikingly 

 shown in the Rio Jequetinhonhaand other rivers, in which, 60 miles below Diamantma, only 

 very small stones are to be found. In these rivers the material of the diamantiferous deposit 

 is much rounded, more so than in others ; the edges and corners of the diamonds are also 

 considerably worn. The fact that the stones diminish in size the farther down the river 

 they are found can easily be explained, when it is considered that material transported by 

 water becomes more and more worn as the distance over which it travels increases, 

 and, moreover, that the smaller the stones become, the more easily are they transported 

 by water and the greater will be the distance they are carried from their original 

 situation. 



The diamantiferous debris which lies in the beds of the rivers and the bottom of the 

 valleys consists mainly of rounded fragments of rocks and quartz brought down by the 

 streams and rivers from their sources and upper reaches. This debris is usually mixed with 

 clay to a greater or less extent, the resulting material in the state in which it is worked for 

 diamonds being known to the diamond-diggers as cascalho. This is usually loose and 

 incoherent, showing no signs of bedding ; at times, however, it has a firmer consistency due- 

 to the presence of the clay. The upper portion of the mass is sometimes bound together,, 

 to a greater or less depth, by a ferruginous cementing material so as to form a conglomerate. 

 TTiis conglomerate, consisting largely of rounded quartz grains and pebbles, occurs in 

 extensive beds or in isolated blocks known as " tapanhoacanga," or " canga," which may 

 enclose crystals of diamond. Such a fragment of conglomerate, with a diamond embedded in 

 it, is represented in Plate I., Fig. 1 ; similar specimens are often exhibited in collections as. 

 examples of the occurrence of diamond in its mother-rock, a view which, as we have seen, 

 is incorrect. 



The " cascalho " of the diamond-diggers thus contains the diamond with its associated 

 minerals as a finer constituent, and rounded rock-fragments as coarser material, the whole 

 being intermixed with clay or with limonite, which may cement the material together into a. 

 more or less firm mass. This material lies in the beds of the water-courses resting 

 immediately on the solid rocks beneath ; in a few instances, however, the rich diamond- 

 bearing " cascalho," the " cascalho virgem " of the Brazilians, extends upwards to the 

 surface through the whole of the fluviatile deposits. It has a very variable thickness, and 

 is usually covered by a layer of material from which diamonds are absent, the so-called 

 " barren cascalho " ; this upper layer varies in thickness, from a few centimetres to twenty or 

 thirty metres ; in its lower portion it usually consists of an accumulation of larger rock- 

 fragments. The " barren cascalho " is constituted of materials similar in character to those^ 



