O Solo da Amazônia

Resumo

The author discourses in a general manner upon the soil of the Amazon region- Amazonia -,showing its most characteristic features, in accordance with the descriptions of the principal observers of that region.

            If we Iook upon the Amazonian plain as consisting of two gently sloping surfaces, one from the North towards the South and the other from South to North, we shall find at their

Intersection the valley of the great river, many kilometres wide, and beaded to a large extent by a chain of lakes. The plain is bounded in the North by a series of elevations made un of a plateau of prophiry-bearing sand layres, and by formations of gneissic granite from the mountains which separate Brazil from the Guianas. The Southern boundary is less precise and the ground gradually slopes almost unnoticed up to the sandy flot-topped hills of the North of Mato Grosso.

            To the West of the Acre Territory begin the first undulations of the Andine system, though the plains reach into Perú and Colombia without any noticeable elevations.

            The most characteristic feature of the Amazon soil is the existence of an enormous covering of tertiary rocks and quaternary alluvial soil over the plains. The so-called firm Iands, which form a higher revel and are free from the usual floods, are generally formed of tertiary horizontal Iayers and of red and stained clays belonging to the well-known formation of embankments. The lower lands of the marshes, those occupied by igapós (recent alluvials) and lakes, are generally formed of recent and quaternary alluvion, made up of sandy clays and silt.

            North and South of the Amazon River appear strips of paleozoic terrain, becoming older as they recede from the axis of the river. Thus first appears the carno niferous soil, then the Devonian, the Silurean and, finally, rocks, possibly Algonkian, at some points. To the North and south of the Paleozoic strip the Archaean comes to the surface greatly eroded and peneplanated and covered with alluvium to a great extent. There is a remarkable area to the North of the Amazon River where may be noted a vast overflow of acid eruptive soil, forming the field of felsitic porphiry, which covers large tracts of ground in the high table-lands Northwards.

            When dealing with the mineral resources of the Amazon soil the author refers to the

presence of gold in the rivers of the frontier with British Guiana, diamonds and diatomite in

the valley of the Rio Branco. He briefly discurses the question of the possibility of the existence of coal and petroleum and ponders upon the great difficulties which the Amazon Rivers offers to soil-prospecting and that only modem geophysic and geochemical processes can be efficiently used for the solution of the great problems of the Amazonian sub-soil.