The rivers in the environs of Diamantina rim at the bottom of deep and narrow gorges that have been scooped out to depths of 300 or 400 meters through the denuded plateau in whose center stands the city of Diamantina.  In the bed of these rivers, in places where they have not yet been worked, there may be found, underneath a stratum of modern sand, another of rocks, and finally a diamondiferous deposit of rounded pebbles, mixed with sand.  This gravel, which is characterized in the first place by the fact that all its elements are rounded, and next by the presence of a large number of minerals (among which the most important are all the oxides of titanium, different oxides of iron, tourmaline, and a whole series of hydrated phosphates of complex composition), is called in the language of the country cascalho.  It is the matrix of the diamond, and the latter is extracted from it by washing.  It is arranged in roundish masses upon the beds of the rivers, and is met with at depths ranging from a few decimeters up to 25 and 30 meters.

The same material, with the same name, is also found deposited at all heights upon small terraces at the sides of the valleys through which the rivers flow.  It is coarser and less rolled, and has very likely been deposited by risings of the rivers during the period when the valleys were being formed.  These deposits bear the name of gupiarras.  Finally, it is found in a still coarser state, mixed with red earth and deposited in horizontal strata upon the upper plateau.  It is then called gorgulho.

Of these different deposits, the most important are those of the river beds, the material here having undergone a true mechanical preparation and being richer.  These are the deposits that have been the object of the most important exploitations.

The year is divided into two distinct seasons—­the dry, from May to September, during which rain is exceptional, and the rainy, from October to April.  As water is necessary for all the operations, no work can be done upon the high plateaux except through rain water stored up in large reservoirs.  These beds form what are called the “rainy season washings.”  In the rivers the working of the beds requires a preliminary drying, which is effected by diverting the river’s course.  Now in all this rocky and denuded region the water that falls runs immediately to the river, and causes terrible freshets therein; so operations capable of keeping the bed dry would be out of proportion to the probable results of the exploitation, whence it follows that the latter is only possible in dry weather, and these deposits are therefore called “dry season washings.”

These deposits are still worked in our day as they were in the time of the Portuguese.  In order to dry the bed a dam is constructed, and the river is either diverted into a plank flume supported by piles, or into a canal dug along the shore, or by means of tight walls, according to the lay of the place.  The second process, which is preferable to the first, is in fact impossible when the river runs, as is often the case, in a narrow, abrupt, walled channel.  These works are sometimes very important.  In 1881, the Acaba Mundo flume was 140 meters in length and 5.2 m. wide, and, with a velocity of 2.25 m., discharged 4,500 liters per second; still longer ones might be cited that discharged as much as 8,000 liters.