DIAMOND: OCCUKRENCE IN BRAZIL 167 



Thus at Sao Joao da Chapada, and elsewhere on the plateau, we see the diamond still 

 at the place and in the rock in which it was originally formed, though the latter has been, 

 in part at least, altered by weathering to a soft clayey mass. In such a case we may 

 distinguish the deposit as original or primai-y. 



Other plateau-deposits, where the rock-fragments are but slightly, if at all, rounded, 

 must have been laid down at an early period when the plateau had been eroded by water- 

 courses to only a slight extent and before the present valleys came into existence. The 

 diamonds and their associated minerals were indeed carried away by water from the 

 disintegrated mother-rock, in which at Sao Joao they still remain, but they were 

 re-deposited at spots not very far distant, as is proved by the fact that they are so little 

 water-worn. The diamonds and other minerals were probably re-deposited on the floors 

 of shallow lakes, a hypothesis which would account for the bedding of the material in 

 which they occur. Such deposits are described as secondary or derived. 



The slow but never ceasing erosion of the plateau by streams and rivers resulted in 

 the course of ages in the formation of the valleys of the present day. Here, high up on 

 the sides of the valley, the most ancient deposit marks the level of the original river-bed. 

 The material for this deposit was derived partly from the primary or original deposit and 

 partly from the secondary plateau-deposit. Some of the diamonds of the oldest valley- 

 deposits have therefore changed their situation twice, and the material, having undergone a 

 second transportation, is therefore more appreciably rounded. As the rivers carved out for 

 themselves deeper and ever deeper channels, so fresh deposits were laid down, the material 

 of each successively lower level being more and more water-worn, until the wearing dowji 

 process culminates in the much rounded material of the present river-bed. The older 

 valley-deposits are now to be found forming the terraces high up on the sides of the 

 present valleys, while those in the present river-bed itself constitute the deposits described 

 above as the river-deposits. 



What has been said above with regard to the original mode of occurrence of 

 diamond in Minas Geraes may be summarised as follows : The home of the diamond is 

 located in those portions of the plateau-deposits in which the diamantiferous rivers take 

 their origin. The diamonds gradually decrease in size and number and finally disappear 

 further down the valleys. The rock, which is in situ in the neighbourhood of the plateau- 

 deposits, is everywhere itacolumite, interbedded with schists and covered by the younger 

 quartzite. This itacolumite is evidently the source from which the diamonds and the 

 material in which they are found have been derived. So much was established by L. von 

 Eschwege as early as the beginning of the nineteenth century ; this observer noticed that in 

 the Diamantina district diamonds were found only in those rivers, such, for example, as the 

 Rio Jequetinhonha, which flow down the western slopes of the Serra do Espinha^o which 

 are formed of itacolumite, and that, on the other hand, no diamonds are found in those 

 rivers, such, for example, as the Rio Doce and its tributaries, which rise on the eastern 

 side of this range, these slopes being formed of gneiss, mica-schist, &c. As has been 

 already pointed out, those rivers which flow through strata composed of itacolumite are 

 diamantiferous, while in those flowing through districts from which itacolumite is 

 absent no diamonds are found. 



The occurrence of the minerals associated with diamond, especially the most important 

 of them, such as quartz (rock-crystal), oxides of iron and of titanium, tourmaline, &c., is 

 also confined to the itacolumite ; they do not, however, occur embedded in the rock but in 

 the veins, consisting mainly of quartz, by which it and the interbedded schists are penetrated. 

 The circumstance that the diamond is invariably associated with these minerals, and with 



