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Updated: May 5, 2025
All the rocks were igneous; we had left the sandstone that had marked the course of the Atbara and the valley of the Settite as far as Ombrega, and I was extremely puzzled to account for the presence of the pure white and rose-coloured limestone that we had found only in one place Geera.
But such is the tremendous power of the Atlantic billows when impelled by westerly winds, that to their agency must be mainly attributed the small size of the volcanic land-surfaces as compared with their original extent, and the formation of those grand headlands which are presented by the igneous masses of Skye, Ardnamurchan, and Mull towards the west.
On the other hand, Becker supposes the concentration to have been effected by surface waters flowing laterally through the igneous rocks, gathering the precious metals and depositing them in the fissure, as lateral secretion produces the accumulation of ore in the limestone of the lead region.
With regard to the evaporation or separation of the aqueous solvent, this may be easily conceived according to the igneous theory; but, the aqueous theory has not any means for the producing of that effect in the mineral regions, which is the only place we are here concerned with.
I did not visit the others, but I was informed that there are seams in the independent native territory as well as in that of the Portuguese. That in the Nake is in the Banyai country, and, indeed, I have no doubt but that the whole country between Zumbo and Lupata is a coal-field of at least 2-1/2 Deg. of latitude in breadth, having many faults, made during the time of the igneous action.
It was not till geologists ceased to discuss the condition of the original nucleus of the planet, whether it was solid or fluid, and whether it owed its fluidity to aqueous or igneous causes, that they began to achieve their great triumphs; and the vast progress which has recently been made in showing how the living species may be connected with the extinct by a common bond of descent, has been due to a more careful study of the actual state of the living world, and to those monuments of the past in which the relics of the animate creation of former ages are best preserved and least mutilated by the hand of time.
These older rocks are still as flat and horizontal as when first formed; yet, since their origin, not only have most of the actual mountain-chains been uplifted, but some of the very rocks of which those, mountains are composed have been formed, some of them by igneous and others by aqueous action.
Modern researches have shown, too, that these metamorphic rocks are not, as was once supposed, all of the same age. Besides primary and secondary strata which have been transformed by igneous action, there are similarly-changed deposits of tertiary origin deposits changed, even as far as a quarter of a mile from the point of contact with neighbouring granite.
Piquillacta is a large mass, nearly five miles long and two miles wide, rising to a point of 2000 feet above the Huatanay and Vilcanota rivers, it has no streams, brooks, or springs. It is an isolated, extinct volcano surrounded by igneous rocks, lavas, andesites, and basalts.
Besides the numberless dislocations of strata, the ejections of igneous matter, the propagation of earthquake vibrations thousands of miles around, the loud explosions, and the escape of gases; there would be the rush of the Atlantic and Pacific Oceans to supply the vacant space, the subsequent recoil of enormous waves, which would traverse both these oceans and produce myriads of changes along their shores, the corresponding atmospheric waves complicated by the currents surrounding each volcanic vent, and the electrical discharges with which such disturbances are accompanied.
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