160 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



sedimentary rock rather than as belonging to the crystalline schists ; it is very abundant in 

 the Serra Itacolumi, in the southern part of Minas Geraes, and has thus come to be known 

 as itacolumite. Interbedded with it are clay-slates, and various schists such as mica-schist, 

 hornblende-schist, haematite- schist, &c. The rock is penetrated for short distances by veins 

 which usually contain crystals of quartz. The beds of itacolumite and associated rocks, 

 together with the underlying gneiss, mica-schist, and hornblende-schist, are usually inclined 

 at a steep angle. 



On the mountain-tops the itacolumite is overlain unconformably by a younger quartzite. 

 the bedding of which is less steeply inclined than is that of the underlying rock. It is very 

 similar in appearance to the itacolumite, and in places it merges into a conglomerate just as 

 the itacolumite does. From the fact that irregular and angular projections of the lower 

 rock are covered by the younger quartzite, it is evident that . the two rocks are perfectly 

 distinct, and probably belong to very different periods. 



At some places, and conspicuously so in the basin of the Rio de Sao Francisco, the beds 

 of itacolumite are associated with slates and limestones, in which fossils of Silurian and 

 Devonian age are found. These slates and limestones have no direct bearing on the occurrence 

 of diamonds, since, as we shall see later, the itacolumite must be regarded as the diamond - 

 bearing rock ; they may, however, serve to determine the geological age, at present unknown, 

 of the itacolumite when its relation to these rocks has been made out, which has not yet been 

 accomplished. 



We have already seen that the mode of occurrence of the diamond differs in each of 

 these districts. TTiree kinds of diamantiferous deposits are distinguished according to their 

 situation, whether on the plateau or in the valley, and, in the latter case, whether above or 

 below the present high-water level. These are known respectively as river-deposits, valley- 

 deposits, or plateau-deposits, according as they are found in the existing water-courses within 

 the present limits of high water, on the sides of the valley above high- water level, or covering 

 more or less large areas on the summits of plateaux. 



Both the river-deposits and the valley-deposits are without exception constituted of 

 sands, the plateau-deposits also having in part a similar constitution ; these sands, or alluvial 

 deposits, consist of debris which has been transported by water, and which contains more or 

 less rounded rock-fragments, among which the diamonds occur singly and isolated. The 

 amount of rounding which the rock-fragments have undergone may be regarded as indicating 

 the distance to which they have been transported from their original situation. In places, 

 however, the rock-fragments of the plateau-deposits show no trace of having been water- 

 worn ; when this is the case, such deposits have undoubtedly been formed on the spot they 

 now occupy, and consist usually of much weathered rock-masses, as will be shown in a special 

 description of certain plateau-deposits. 



An attentive consideration of the distribution of the three classes of deposits leads to 

 the recognition of a certain connection between them which is of some interest. The various 

 diamond-bearing districts of the plateau are at the same time the collecting grounds of the 

 diamantiferous streams and rivers ; it is a natural conclusion then that the stones now found 

 in the sands of the river valleys have been carried there, together with sand, gravel, and other 

 debris, from their original situation on the plateau by these same rivers and streams. This 

 is especially the case in the neighbourhood of the town of Diamantina, which stands on a 

 plateau, the surface beds of which consist of diamond-bearing rock. The rivers which have 

 their sources in these rocks are in their lower courses rich in diamonds, whereas in other 

 rivers, such, for example, as the Rio Doce and its tributaries, which rise among rocks from, 

 which diamonds are absent, no diamonds are to be found. 



