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Updated: May 21, 2025
Though it be certain, that no scientific observer has hitherto found at Teneriffe primitive strata, or even those trappean and ambiguous porphyries, which constitute the bases of Etna, and of several volcanoes of the Andes, we must not conclude from this isolated fact, that the whole archipelago of the Canaries is the production of submarine fires.
The foundation rock is sometimes metamorphic schist, or gneiss, at other times sandstone referred by Hislop to Jurassic age; and in no single instance has a volcanic crater or focus of eruption been observed. But outside the central trappean area volcanic foci are numerous, as in Cutch, the Rajhipla Hills and the Lower Narbudda valley.
The rocks of this part of the country are chiefly trappean; in the immediate neighbourhood of Portland, they consist of limestone, ferruginous sandstone, and trap.
To a certain extent, however, there was a real distinction between the trappean formations and those to which the term volcanic was almost exclusively confined.
Their resemblance to the great trappean series of Western India, even in minute particulars, is referred to by Mr. Blanford, who suggests the view that they belong to one and the same great series of lava-flows extruded over the surface of this part of the globe. This view is inherently probable. Beds of volcanic ash or breccia also frequently occur, and often contain augite crystals.
This information led me into an error. As the coasts of Portugal consist of basalts covering calcareous rocks containing shells, I imagined that a trappean formation, like that of the Vicentin in Lombardy, and of Harutsh in Africa, might have extended from the banks of the Tagus and Cape St.
Numerous dikes likewise consist of this greenstone porphyry; others are formed of various fine-grained trappean rocks; but very few of claystone porphyry: I saw no true basaltic dikes.
Even in regions such as the Alps, where some masses of granite and gneiss can be shown to be of comparatively modern date, belonging, for example, to the period hereafter to be described as tertiary, they are still UNDERLYING rocks. They never repose on the volcanic or trappean formations, nor on strata containing organic remains. They are HYPOGENE, as "being under" all the rest.
By what train of investigations were geologists induced at length to reject these views, and to assent to the igneous origin of the trappean formations? By an examination of volcanos now active, and by comparing their structure and the composition of their lavas with the ancient trap rocks.
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