Basaltos

Dark volcanic rocks, compact and hard, formed of labrador feldspar and augite. They are relatively poor in silica, but contain more lime and magnesia than trachyte, which is rich in silica.

According to the geologist James Smith, of Jordan Hill, in Madeira, we know of compact basalt (living stone or masonry), vesicular basalt (hard stone) and scoriaceous basalt. Tuffs often alternate with basalts, and it is common to see compact basalts resting on layers of scoriaceous basalt.

Compact basalt, when in thick layers, often splits vertically into columns of polygonal section, as is frequently seen in quarries, and as can also be observed near Gorgulho, Tem te não Caias (Porto da Cruz), and Faial.

Compact basalt is used in all kinds of constructions, but it is unsuitable for carving due to the hardness of its grain.

Vesicular basalt is mainly found in the vicinity of Câmara de Lobos, and is used throughout the island for door thresholds, corners, steps, etc..

Scoriaceous basalt appears everywhere, often with compact basalt in between, but it is not used in construction.

Basalts, very common in the lower region of Madeira, and also found in Porto Santo and the Desertas, generally contain crystals of augite and olivine, the former being black and the latter greenish. They often appear in dykes, vertical or oblique, crossing layers of conglomerates or basaltic rocks that have split.