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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 firmness. This material lies in the beds of the water-courses resting immediately on the solid rocks beneath; in a few instances, however, the1 rich diamond bearing "cascalho", the "cascalho virgem" of the Brazilians, extends upwards to the surface through the whole of the alluvial 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 centimeters to twenty or thirty meters; 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 of the more deeply situated diamond-bearing layer, to reach, which it is necessary to divert the water that flows over the barren layer. Although the diamantiferous "cascalho" is spread fairly uninterruptedly over long stretches of the beds of streams and rivers, yet its distribution over the whole course of the river is by no means regular. Here it may be accumulated in masses of great thickness, there to only a sparing amount, while in a third place it may be altogether absent. Moreover, the number of diamonds present in the material varies in different rivers, and in different parts of the same river; it is recorded of certain rivers in the Diamantina district, however, that the diamonds were so regularly distributed through the "cascalho" that it was possible to estimate with accuracy the weight in carats of the precious stone which a certain amount of this material would yield; this case is, however, very exceptional.

A large accumulation of especially rich "cascalho" at one particular point is the result of the presence of certain conditions, which exist only at that point; such an accumulation is sought for with eagerness. In the beds of the rivers cylindrical holes of greater or less depth are sometimes bored in the solid rock by the action of the running water, such potholes or "giants' kettles" being formed in the same way in many other parts of the world. In addition to these, long channels, hollowed out of the bed of the water-course, and following its course for a certain distance, or running obliquely across it, are also to be met with, and are sometimes known as "subterranean cañons". Such hollows in the bed of a stream occur where the water has passed over softer beds, these being worn away to a greater depth than are the surrounding harder rocks. The hollows so formed may be small or of considerable size, and are often filled up with a specially rich "cascalho." In a small hollow in the bed of the Ribeirao do Inferno, which joins the Jequetinhonha near Diamantina, 8000 to 10,000 carats of diamonds were found, the neighboring part of the riverbed being very poor. Again, in a small pothole in the bed of the Rio Pardo, diamonds to the weight of 180 carats were obtained in the short space of four days. Again, the three mines in the valley of the Jequetinhonha, which have been specially prolific, namely, S. Antonio, with Canteiras above, and Acaba Mundo below, the mouth of the Ribeirao do Inferno, were worked in depressions of the nature of channels or "subterranean cañons" in the river-bed.

The valley-deposits ("gupiarras" of the Brazilians) are, as a rule, of small extent; they are formed of the same material as are the river-deposits, and the diamonds are associated with the same minerals. This deposit is also known as "cascalho," and sometimes also as "gurgulho"; the latter term, however, is more often applied to the material of the plateau-deposits. The valley-deposits also follow the direction of the present water-courses, being situated at the sides of the valley above the present high-water level; they are, as a matter of fact, river-deposits laid down at a time when the bed of the river had not been excavated to the extent it now is. In many cases the successive levels of the former beds of the river are marked out on the sides of the valley by a series of such deposits or river terraces.

The material of these terraces is much less worn than is that in the bottom of the valley. As a general rule, it is found that the rounding of the rock-fragments is the more pronounced the lower is the level of the terrace in which they occur, and further that material of any given terrace becomes more worn the further it is deposited from the source of the river. At the bottom of the same valley in which river-terraces are to be seen is the present bed of the river with its deposits, the material of which is more worn and rounded than that of any of the valley-deposits. It is therefore possible by means of this difference 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 colored layer of fine sand mixed with clay; this is not of any great thickness, and is called "barro." It also contains diamonds, and passes gradually, with no sharp line of demarcation, into 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. A layer of red muddy earth often, but not invariably, covers it. 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, between the Rio Jequetinhonha and the town of Diamantina, lying about due east of the latter, were the rich mines Bom Successo and Boa Vista. On the plateau southwest of Diamantina, and between the basins of the Rio Pinheiro and the Rio Pardo Pequefia, were the mines La Sopa and Guinda, 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 especially noteworthy deposits of Sao Joao da Chapada, which will be described below. A little to the south of this place, in the neighborhood of the source of the Rio Ouro Fino, are the diggings of La Chapada, which were formerly very rich, and are now 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.


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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011