Rochas-mater da "terra roxa"

Abstract

Physico-chemical and mineralogical studies of the soils of the São Paulo State by the Soil Division of The Agronomical Institute proved the existence of different violet soils in South-Brasil and its origin from basaltic rocks. The Brasilian denomination "terra roxa" is already many times translated to "red soil", which is inexact, because the colour "roxa" corresponds to "violet" in English. We must insist on the perfection of Brasilian expression, which gives the shortest and the best characteristic of the true violet soil, derived through the decomposition of basalts and diabases in the São Paulo State. The term "red soil" originated much confusion, because there are in this state many "red soils" of different origins, but the true violet soil is quite unique. The violet colour of this soil appears very beautifully on the clean fields above the diabase hills at the distance of some hundreds of meters. In the state of complete dryness the violet soil becomes coffee brown, but never gets a red colour. The violet soil is the best soil of South America, on the contrary the red soils, which are mostly lateritic, are bad soils. Some exceptions, do exist, of course. The basalts produce laterites and other red soils of better qualities. With the purpose to contribute to the study of the violet and the red soils the present essay was made, describing the basic rocks of South-Brasil, as mother rocks of soils. The basic monograph of Djalma Guimarães "Magmatic Province of South-Brasil" (5), many times mentioned in the petrographic literature, definitively established the principal types and the mineral-components of triassic basic rocks in South-Brasil. We have found some varieties of basic rocks, not y