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Updated: June 12, 2025
I noticed on the north-west extremity of the basin of the West Indies that the secondary formations dip towards south-east; along the coast of Venezuela rocks of gneiss and primitive mica-slate dip to north-west. The basalts, amygdaloids, and trachytes, which are often surmounted by tertiary limestones, appear only towards the eastern and western banks.
The two principal members of the Plutonic family of rocks are Granite and Syenite, each of which, with their varieties, bear very much the same relation to each other as the trachytes bear to the basalts. Granite is a compound of feldspar, quartz, and mica, the feldspars being rich in silica, which forms from 60 to 70 per cent of the whole aggregate.
These five rocks, dispersed over the whole globe, charged with oxidulated and titanious iron, are probably of similar origin. See MacCulloch in Edinburgh Journal of Science 1824 July pages 3 to 16. No volcanic rocks of a more recent period have hitherto been discovered in the island of Cuba; for instance, neither trachytes, dolerites, nor basalts.
At noon, we reached a small town called Concordia, where the houses were larger and better built than those in the small towns of Segovia. The church, on the other hand, was an ugly barn-like building, apparently much neglected. The rocks were trachytes, and the soil seemed fertile, but there was very little of it cultivated. Many of the men we met wore long swords instead of the usual machetes.
The two principal families of trappean or volcanic rocks are the basalts and the trachytes, which differ chiefly from each other in the quantity of silica which they contain. The basaltic rocks are comparatively poor in silica, containing less than 50 per cent of that mineral, and none in a pure state or as free quartz, apart from the rest of the matrix.
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