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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.
Diorite forms the great mass of this stratum; it is of a dark green colour, granular, with small grains, and destitute of quartz; its mass is formed of small crystals of felspar intermixed with crystals of amphibole. This rock of diorite is covered at its surface, by the effect of decomposition, with a yellowish crust, like that of basalts and dolerites.
The study of volcanoes may be divided into two distinct branches; one, simply mineralogical, is directed to the examination of the stony strata, altered or produced by the action of fire; from the formation of the trachytes or trap-porphyries, of basalts, phonolites, and dolerites, to the most recent lavas: the other branch, less accessible and more neglected, comprehends the physical relations which link volcanoes together, the influence of one volcanic system on another, the connection existing between the action of burning mountains and the commotions which agitate the earth at great distances, and during long intervals, in the same direction.
These dolerites are cut through by intrusive syenite, and this syenite, in its turn, is again cut and penetrated by feldspar porphyry, the base of which consists of petrosilex, or a mixture of orthoclase and quartz. All these trap- rocks appear to be of Laurentian date, as the Cambrian and Huronian rocks rest unconformably upon them. General Aspect of Plutonic Rocks. Granite and its Varieties.
From this place to the mines the rocks were soft decomposing dolerites, with many harder bands of felsite, and, occasionally, plains composed of more recent trachytic lavas. We passed through another weedy, dilapidated town, called Condego, where they have a singular custom at their annual festival held on the 15th of May.
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