Fig. 5.—Theoretical view of a basaltic plateau.
Basaltic Formations.
Basaltic eruptions seem to have occurred during the Secondary and Tertiary periods. Basalt, according to Dr. Daubeny,[20] in its more strict sense, “is composed of an intimate mixture of augite with a zeolitic mineral, which appears to have been formed out of labradorite (felspar of Labrador), by the addition of water—the presence of water being in all zeolites the cause of that bubbling-up under the blow-pipe to which they owe their appellation.” M. Delesse and other mineralogists are of opinion that the idea of augite being the prevailing mineral in basalt, must be abandoned; and that although its presence gives the rock its distinctive character, as compared with trachytic and most other trap rocks, still the principal element in their composition is felspar. Basalt, a lava consisting essentially of augite, labradorite (or nepheline) and magnetic iron-ore is the rock which predominates in this formation. It contains a smaller quantity of silica than the trachyte, and a larger proportion of lime and magnesia. Hence, independent of the iron in its composition, it is heavier in proportion, as it contains more or less silica. Some varieties of basalt contain very large quantities of olivine, a mineral of an olive-green colour, with a chemical composition differing but slightly from serpentine. Both basalts and trachyte contain more soda and less silica in their composition than granites; some of the basalts are highly fusible, the alkaline matter and lime in their composition acting as a flux to the silica. There are examples of basalt existing in well-defined flows, which still adhere to craters visible at the present day, and with regard to the igneous origin of which there can be no doubt. One of the most striking examples of a basaltic cone is furnished by the mountain or crater of La Coupe d’Ayzac, in the Vivarais, in the south of France. [Plate II.], on the opposite page, gives an accurate representation of this curious basaltic flow. The remnants of the stream of liquefied basalt which once flowed down the flank of the hill may still be seen adhering in vast masses to the granite rocks on both sides of a narrow valley where the river Volant has cut across the lava and left a pavement or causeway, forming an assemblage of upright prismatic columns, fitted together with geometrical symmetry; the whole resting on a base of gneiss. Basaltic eruptions sometimes form a plateau, as represented in [Fig. 5], where the process of formation is shown theoretically and in a manner which renders further explanation unnecessary. Many of these basaltic table-lands form plateaux of very considerable extent and thickness; others form fragments of the same, more or less dislocated; others, again, present themselves in isolated knolls, far removed from similar formations. In short, basaltic rocks present themselves in veins or dykes, more or less, in most countries, of which Central France and the banks of the Rhine offer many striking examples. These veins present very evident proofs that the matter has been introduced from below, and in a manner which could only result from injection from the interior to the exterior of the earth. Such are the proofs presented by the basaltic veins of Villeneuve-de-Berg, which terminate in slender filaments, sometimes bifurcated, which gradually lose themselves in the rock which they traverse. In several parts of the north of Ireland, chalk-formations with flints are traversed by basaltic dykes, the chalk being converted into granular marble near the basalt, the change sometimes extending eight or ten feet from the wall of the dyke, and being greatest near the surface of contact. In the Island of Rathlin, the walls of basalt traverse the chalk in three veins or dykes; the central one a foot thick, that on the right twenty feet, and on the left thirty-three feet thick, and all, according to Buckland and Conybeare, within the breadth of ninety feet.
Fig. 6.—Basalt in prismatic columns.
