Vietnam or Thailand ? Vote for the TOP Country of the Week !
Updated: September 3, 2025
These injections never reached to the surface probably never beyond the area of heated rocks; so that there have been no overflows from them, and they have only been exposed by subsequent great upheaval and denudation.
Such are the hills of Monte Alegre, of Obydos, Almeyrim, and Cupati, as well as the lower ridges of Santarem. This escape of the waters did not, however, entirely empty the whole basin; for the period of denudation was again followed by one of quiet accumulation, during which was deposited the ochraceous sandy clay resting upon the denudated surfaces of the underlying sandstone.
The variety we find, both as to the contours and the collocation of the islands, is due chiefly to differences in the structure and composition of their rocks, and the unequal glacial denudation different portions of the coast were subjected to. This influence must have been especially heavy toward the end of the glacial period, when the main ice-sheet began to break up into separate glaciers.
After this denudation, which may have accompanied the emergence of the land, the country was inhabited by the primitive people who fashioned the flint tools.
The Mississippi is also more likely to afford us a fair test of ordinary denudation, because, unlike the St. Lawrence and its tributaries, there are no great lakes in which the fluviatile sediment is thrown down and arrested in its way to the sea.
Much useful instruction may therefore be gained from the exploration of a country like Auvergne, where the superficial gravel of very different eras happens to have been preserved and kept separate by sheets of lava, which were poured out one after the other at periods when the denudation, and probably the upheaval, of rocks were in progress.
In Patagonia, we have seen that the elevation has been equable, and the periods of denudation synchronous over very wide spaces of coast; on the shores of the Pacific, owing to the terraces chiefly occurring in the valleys, we have not equal means of judging on this point; and the very different heights of the upraised shells at Coquimbo, Valparaiso, and Concepcion seem directly opposed to such a conclusion.
The thrust planes, he says, are with much difficulty distinguished "from ordinary stratification planes, like which they have been plicated, faulted, and denuded. Here and there, as a result of denudation, a portion of one of them appears capping a hilltop.
For a long series of subterranean movements must occur before such rocks can be uplifted into the atmosphere or the ocean; and, before they can be rendered visible to man, some strata which previously covered them must have been stripped off by denudation.
The cause of these rich deposits near the surface does not appear to me to be that the lodes originally, before they were exposed by denudation, contained more gold in their upper portions than below, but to be the effect of the decomposition and wearing down of the higher parts, and the concentration of the gold they contained in the lode below that worn away.
Word Of The Day
Others Looking