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inches of rainwater; when, in addition, one has in view the horizontal flatness and the porosity of the predominant structure of the stone, the gypsum

Gypsum

conglomerate and the keuprico gres, with which one can explain why water flows from the soil in so many points. Any gathering of water, however small in the vicinity of the river, by the great extent of the flat banks, then grows greatly with tributaries from afar, for in the whole Amazon basin it is rarer to find a stream or pond than a river and a deep lake. Most of these lakes are in communication, by means of canals, with the master rivers or their tributaries, originating, perhaps, from the reciprocal floods.
   
Among these accumulations of water, there are many that are the so-called "black waters", equal to those of the Rio Negro, which, seen in a glass, present all the shades from light yellow to amber and brown. That the origin of such sewage is entirely related to the local condition, is verified by the color diversity of the waters, within a perimeter of a few hours of extension. Everywhere I made the observation that these black waters disperse the light more strongly than white water, which gives rise to the opinion that they contain, in dissolution, some combustible material (bitumen, peat, or other essential matter?). It is known, moreover, that the lakes of black water are deeper, of fresher water, and are more constant in quantity than those of white water, which overflow and which are also more often flooded by the next rivers. For this reason, the shores of the black lakes are healthier; and are also less devastated by insects, because they are sandy and do not shelter algae larvae and eggs. The sight of the pirizals or reeds (from the word Tupi piri) led me to the assumption that these black lakes owe their existence, at least in part, to the extractive substance of forests, around which springs sprang up, that finally buried themselves deeply. These lakes and lagoons are, in various ways, in communication with the Amazon and its tributaries, because of :
a) an independent spill of surplus waters in the rivers, in the time of the rains;
b) or receiving spills from those, that like arms deviate from the main direction by a division of the bed (Humboldt, V, 404);
c) or, because they are in the way of a tributary, which runs to the master-river, and is crossed by the master-river.
d) or because an even more complicated condition appears when the lakes receive small tributaries of their own, as well as being crossed by some arm of a large tributary, on the way to the main river.

All these cases occur in the terrain because of its general flatness and in the most diverse directions, where it is cut by shallow valleys, favoring in different ways the movement of the waters towards the deepest valley line, towards the Amazon itself.

The lowering of the banks also allows for many communications between tributaries of the Amazon above their mouths, so that the whole territory seems cut off in several directions by channels, such as the Netherlands. An example of this kind to the west also offers the problematic channel, Cano de Abusau, in which Fr. Narciso Girbal, in 1794, seems to have verified a communication between the Ucaiale and the Javari. In this sense, it is also worth noting here the links between the Purus and Madeira in two points, forty hours apart, made by the Rio Capana, and by the very branched system of the Rio Uautas. I must also note here a constant kind of distribution of water produced by the localities, such as the double mouths or bifurcation of the rivers, which, besides the main mouth, communicate with the river and the secondary river.

All these additional mouths and tributaries, I would distinguish as active and passive: the active proceeds exclusively from the tributaries, for these: a) separate the bed through the uneven low land, which has several low hills; b) or, with the accumulation of rolled pebbles and basins, they form deltas in the primitive river’s mouth. An example of this is the mouths of the Matari, Arauato and Uatuma rivers on the northern side, and those in the Uraria hole, on the southern side of the river.

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