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Updated: July 23, 2025
Sir A. Geikie has disputed the correctness of the view, which I advocated as far back as 1874, that the trachytic lavas of Antrim are the earliest products of volcanic action; but at the time he wrote his paper on the volcanic history of these islands, it was not known that pebbles of this trachyte are largely distributed amongst the ash-beds which occur in the very midst of the overlying basaltic sheets, as I shall have to explain later on.
The coloured beds under the conglomerate in the valley of Tenuyan, of which traces are seen on the crest of the Portillo, and even the conglomerate itself, may perhaps be synchronous with the tufaceous beds and submarine lavas of the Uspallata range; an open sea and volcanic action in the latter case, and a confined channel between two bordering chains of islets in the former case, having been sufficient to account for the mineralogical dissimilarity of the two series.
I preferred to admit in truth, that this chimney of an extinct volcano, lined with lavas, which are non-conductors of heat, did not suffer the heat to pass through its walls. But without stopping to look up new arguments I simply took up our situation such as it was. "Well, admitting all your calculations to be quite correct, you must allow me to draw one rigid result therefrom." "What is it.
These rocks approach towards true granites in one direction, and through quartz-porphyry and felsite to rhyolite in another probably depending upon the conditions of cooling and consolidation. In their mode of weathering and general appearance on a large scale, they present a marked contrast to the basic lavas with which they are in contact from the coast of L. na Keal to that of L. Buy.
Hot springs rising through alkaline siliceous rocks, such as lavas, often deposit silica in a white spongy formation known as SILICEOUS SINTER, both by evaporation and by the action of algae which secrete silica from the waters. It is in this way that the cones and mounds of the geysers in the Yellowstone National Park and in Iceland have been formed.
Vesuvius, during a long series of years, has also thrown out lavas without leucites: and if it be true, as M. von Buch has rendered very probable, that these crystals are formed only in the currents which flow either from the crater itself, or very near its brink, we must not be surprised at not finding them in the lavas of the peak.
In the absence of British examples of volcanic rocks newer than the Upper Miocene, I may state that in other parts of the world, especially in those where volcanic eruptions are now taking place from time to time, there are tuffs and lavas belonging to that part of the Tertiary era the antiquity of which is proved by the presence of the bones of extinct quadrupeds which co-existed with terrestrial, fresh-water, and marine mollusca of species still living.
These pervious sands, interbedded with the lava, become the aquifers of artesian wells. In places the lavas rest on extensive lake deposits, one thousand feet deep, and Miocene in age as their fossils prove. It is to the middle Tertiary, then, that the earliest flows and the largest bulk of the great inundation belong.
Having thus found a distinguishing character for those fused substances called, in general, Lavas, and having the most visible marks for that which had been actually a volcano, naturalists, in examining different countries, have discovered the most undoubted proofs of many ancient volcanos, which had not been before suspected.
Vincent as far as the Canary Islands; and that the basalts of the Peak might perhaps conceal a secondary calcareous stone. These conjectures exposed me to severe animadversions from M. G.A. de Luc, who is of opinion that every volcanic island is only an accumulation of lavas and scoriae. M. de Luc declares it is impossible that real lava should contain fragments of vegetable substances.
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