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As these intrusive mountain-masses form most of the axes-lines in the Cordillera, whether anticlinal, uniclinal, or synclinal, and as the main valleys have generally been hollowed out along these lines, the intrusive masses have generally suffered much denudation.

Downward afresh in their wild ride, the rainbows of the cascades flying beside them, their afternoon shadows streaming up behind them, darkness beginning to gather in the deeps below them, the mighty mountain-masses around rearing themselves impenetrably in boding blackness and mystery against the yellow gleam, the purple breath of evening wrapping them, the dew again, again the stars, and they camped at the foot of a spur of hills with a waterfall for sentry on their left.

The opening is so narrow that it affords but room for the road and the stream, which is crossed by a bridge of Roman construction, restored by the late emperor Napoleon. On each side the great mountain-masses rise, picturesque, even fantastic, in outline. The heights are inaccessible to any foot but those of the goat and goatherd.

From Combarbala to Coquimbo, I traversed the country in a zigzag direction, crossing and recrossing the porphyritic conglomerate and finding in the granitic districts an unusual number of mountain-masses composed of various intrusive, porphyritic rocks, many of them andesitic. One common variety was greenish-black, with large crystals of blackish albite.

The fact of the general proximity of volcanoes to the sea is one which has frequently been pointed out by geographers, and may now be regarded as being thoroughly established. Many of the grandest mountain-chains have bands of volcanoes lying parallel to them. This is strikingly exhibited by the great mountain-masses which lie on the western side of the American continent.

Central North-America is to an extraordinary degree worked out everywhere in careful detail, in moderate hill and valley, in undulating prairie and fertile plain, not tossed into barren mountain-masses and table-lands, like that vast desert plateau which stretches through Central Asia, not struck out in blank, like the Russian steppes and the South American llanos, as if Nature had wanted leisure to elaborate and finish.

This grand range has suffered both the most violent dislocations, and slow, though grand, upward and downward movements in mass; I know not whether the spectacle of its immense valleys, with mountain-masses of once liquified and intrusive rocks now bared and intersected, or whether the view of those plains, composed of shingle and sediment hence derived, which stretch to the borders of the Atlantic Ocean, is best adapted to excite our astonishment at the amount of wear and tear which these mountains have undergone.

Judging therefore of the thickness of the conglomerate, as seen in the separate mountain-masses, I estimated it at least from one thousand five hundred to two thousand feet. These lower beds are traversed by several dikes of decomposing porphyry. The quartzose strata in one spot were folded into a regular dome.

Gillies, can be traced with little interruption for 140 miles southward to the R. Diamante, where it unites with the western ranges: northward, according to this same author, it terminates where the R. Mendoza debouches from the mountains; but a little further north in the eastern part of the Cumbre section, there are, as we shall hereafter see, some mountain-masses of a brick-red porphyry, the last injected amidst many other porphyries, and having so close an analogy with the coarse red granite of the Portillo line, that I am tempted to believe that they belong to the same axis of injection; if so, the Portillo line is at least 200 miles in length.

Late, therefore, in a geological sense, as must be the age of the main part of the red granite, I can conceive nothing more impressive than the eastern view of this great range, as forcing the mind to grapple with the idea of the thousands of thousands of years requisite for the denudation of the strata which originally encased it, for that the fluidified granite was once encased, its mineralogical composition and structure, and the bold conical shape of the mountain-masses, yield sufficient evidence.