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Updated: June 12, 2025
The alumite of Tolfa, which, since my return to Europe, I have examined on the spot, conjointly with Gay-Lussac, has, by its oryctognostic characters and its chemical composition, a considerable affinity to compact feldspar, which constitutes the basis of so many trachytes and transition-porphyries.
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.
I found at Caracas neither basalt nor dorolite, nor even trachytes or trap-porphyries; nor in general any trace of an extinguished volcano, unless we choose to regard the diabases of primitive grunstein, contained in gneiss, as masses of lava, which have filled up fissures. Mont Blanc and Mont d'Or will not be ranged in one and the same class.
I observed, when speaking of El Dorado de Canelas, the Omaguas and the Iquiare, that almost all the rivers which flow from the west wash down gold in abundance, and very far from the Cordilleras. From Loxa to Popayan these Cordilleras are composed alternately of trachytes and primitive rocks.
Unfortunately, the interpretation of the volcanic phenomena by these two accomplished observers has led them to very different conclusions as regards several important points in the volcanic history of these groups of islands; as, for example, regarding the relative ages of the plateau-basalts and the acid rocks, such as the trachytes and granophyres; again as regards the presence of distinct centres of eruption; and also as regards the relations of the gabbros of Skye to the basaltic sheets.
The earliest volcanic eruptions occurred in the later Eocene epoch and were continued into the succeeding Miocene stage. These consisted of rocks moderately rich in silica, and are grouped under the heads of propylite and andesite. To these succeeded during the Pliocene epoch still more highly silicated rocks of trachytic type, consisting of sanidine and oligoclase trachytes.
A similar disengagement of vapours, joined to the elastic force of the gases, which penetrate strata softened and raised up, appears sometimes to have given great extent to the caverns found in trachytes or trappean porphyries. They are lined with sulphur, and differ by the enormous size of their openings from those observed in volcanic tufas* in Italy, at Teneriffe, and in the Andes.
The central bed allows no short cut across: it is a series of rubbish-heaps, parasitic cones, walls, and lumps of red-black lavas, trachytes, and phonolites reposing upon a deluge of frozen volcanic froth ejected by early eruptions. The aspect was rejoicing as the Arabian desert: I would willingly have spent six months in the purest of pure air.
I am inclined to believe that America, on the east of the Andes, would have burning volcanoes if, near the shore of Venezuela, Guiana and Brazil, the series of primitive rocks were broken by trachytes, for these, by their fendillation and open crevices, seem to establish that permanent communication between the surface of the soil and the interior of the globe, which is the indispensable condition of the existence of a volcano.
Through the base are usually disseminated crystals of glassy feldspar, mica, and sometimes hornblende. Although quartz is not a necessary ingredient in the composition of this rock, it is very frequently present, and the quartz trachytes are very largely developed in many volcanic districts.
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