Lignite

In the Ribeiro do Meio (‘middle brook’), in the valley of São Jorge, at about 300 metres above sea level, two seams or deposits of impure lignite have been known for many years. This lignite is associated with various layers of tuff and clay, in one of which fossil leaves of dicotyledons and ferns were found in abundance. Since a thick layer of basalt and scoria, more than 300 metres high, rests on top of the deposits, it seems evident that abundant land vegetation once existed at the spot, long before the volcanic formations that now cover it.

The nature of the tuffs accompanying the lignite and of some of the nearby agglomerates justifies the assumption, says Lyell, that a series of eruptions once took place close to where the deposits lie. It is therefore not impossible that the lignite and the leaves accumulated in a crater, and that the erosion of the valley later exposed them.

It is said that an analysis of the São Jorge lignite showed 60 per cent carbon and 30 per cent ash.

This lignite, which had lain buried for many years owing to landslips from the ground above, was exposed once again in 1917. See Leaf beds and Phanerogams.