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Updated: May 29, 2025
And had they been contemporaneously deposited, without any such dividing ridge, surely some of the gypseous and other sedimentary matter forming such immensely thick masses in the Cordillera, would have extended this short distance eastwards; and surely some of the Uspallata tuffs and basalts also accumulated to so great a thickness, would have extended a little westward.
Gillan calls somewhat vaguely, mountains of hard ferruginous clay, are nothing but an alluvium which we find at the foot of every volcano. Strata of clay accompany basalts, as tufas accompany modern lavas.
The lavas which are not covered with volcanic ashes remain for ages without any appearance of vegetation. On the African soil excessive heat and lengthened drought retard the growth of cryptogamous plants. The basalts of Graciosa are not in columns, but are divided into strata ten or fifteen inches thick. These strata are inclined at an angle of 80 degrees to the north-west.
What are the affinities which unite the productions of modern volcanoes with the basalts, the phonolites, and those porphyries with bases of feldspar, which are without quartz, and which cover the Cordilleras of Peru and Mexico, as well as the small groups of the Monts Dores, of Cantal, and of Mezen in France?
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 earlier eruptions seem to have been chiefly granitic, though this must not be understood in too wide a sense, since there are granite formations even as late as the Tertiary period; those of the middle periods were mostly porphyries and basalts; while in the more recent ones, lavas predominate.
This, Sloggett explained, was the characteristic flame of both cyanogen and hydrocyanic acid vapour, which, being inflammable, may have become locally ignited in the passage over cities, and only burned in that limited and languid way on account of the ponderous volumes of carbonic anhydride with which they must, of course, be mixed: the dark empurpled colour was due to the presence of large quantities of the scoriae of the trappean rocks: basalts, green-stone, trachytes, and the various porphyries.
If the present volcano has given birth to these basalts, we must suppose, that, like the substances which compose the Somma, at the back of Vesuvius, they are the effect of a submarine effusion, in which the liquid mass has formed strata. The feet of our mules were slipping every moment on beds of stone, which were very steep. We nevertheless recognized the remains of an ancient pavement.
If land predominated, the only monuments we are likely ever to find of Miocene date are those of lacustrine and volcanic origin, such as the Bovey Coal in Devonshire, the Ardtun beds in Mull, or the lignites and associated basalts in Antrim.
The trachytes and basalts of Popayan are separated from the system of the volcanoes of Quito by the mica-slates of Almaguer; the volcanoes of Quito from the trachytes of Assuay by the gneiss of Condorasta and Guasunto.
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