The Geology of Madeira

Abstract

The geology of Madeira has already formed the subject of three communications to this Society, and the present notes are therefore brief. The island, it is well known, is almost wholly composed of basaltic rocks, which rise more or less precipitously from the sea and culminate in a central ridge of some 5000 or 6000 feet in height. Including the Desertas, which are separated by a narrow channel, only 400 feet in depth, its extreme length may be roughly stated at 50 miles, and its greatest breadth at 17 miles; yet so broken is the surface that some 250 miles must be traversed in order to become well acquainted with its varying scenery. The oldest of the basalts are considered to have been of submarine formation, and the only palæontological evidence as to their age consists of a small patch of coralline limestone included towards the north-west of the isle, and referred to the Upper Miocene; while a hitherto undescribed bed, with fossil plants, shows that volcanic activity was continued until the most recent period. There are; however, no traces of either hot springs, fumaroles, or sulphur deposits, though some of the lava-streams appear as unweathered as when newly formed; and immediately west of Funchal there are parasitic cones so little altered that it seems difficult to imagine them completely extinct. Madeira, since its discovery, has been profoundly quiescent; if any movement takes place, it is one of slow elevation.