Notes on the Geology of Madagascar

Abstract

G eneral D escription M adagascar is as yet almost a terra incognita to the geologist, nothing, so far as I am aware, but notices of the most fragmentary kind ever having appeared in regard to its geological features; and, indeed, until the country is surveyed by competent men, we must be content with descriptions of the most general character. In the absence of something more complete, I present the following notes, drawn up from personal observation, to which I have added here and there few remarks taken from other sources, as a slight contribution to our knowledge of the geology of this great island. The central portion Madagascar is generally regarded as consisting chiefly of granite. Mr. Wallace, for instance, in his ‘Island Life’ (p. 384), says of it: “ A lofty granitic plateau, from 80 to 160 miles wide, and from 3000 to 5000 feet high, occupies its central portion, on which rise peaks and domes of basalt and granite to a height of nearly 9000 feet;” and in the same book there is a physical sketchmap in which the whole of the interior of the island, from about 14° to 23° S. lat., is represented as an “elevated granitic region.” Now if we use the terms “granite” and “granitic” in a wide and popular sense, and include in them the various members of the crystalline series of rocks, the description may be regarded as correct; for by far the greater part of the eastern half of Madagascar consists of gneiss