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Updated: May 24, 2025
In these respects there is a wide difference between the gypseous conglomerates and those of the porphyritic-conglomerate formation, in which latter, angular and rounded fragments, almost exclusively composed of porphyries, are mingled together, and which, as already often remarked, probably were ejected from craters deep under the sea.
I even thought, with many other geologists, that obsidian, so far from being vitrified lava, belonged to rocks that were not volcanic; and that the fire, forcing its way through the basalts, the green-stone rocks, the phonolites, and the porphyries with bases of pitchstone and obsidian, the lavas and pumice-stone were no other than these same rocks altered by the action of the volcanoes.
This latter formation, however, here has not been nearly so much metamorphosed as in most parts of Central Chile; it is composed of beds of true purple claystone porphyry, repeatedly alternating with thick beds of purplish-red conglomerate with the well-rounded, large pebbles of various porphyries, not blended together.
These strata are less connected by alternation than by opposition. Porphyritic breccias which envelope the quartzose porphyries. The gem-salt oscillates from zechstein to muschelkalk. Tertiary strata, showing a much smaller number of dicotyledonous plants. These alternations and transitions appeared to me less common in the littoral Cordillera of Venezuela than in the Sierra Parime.
At the period when the claystone and greenstone porphyries nearly or quite ceased being erupted, that great pile of strata which, from often abounding with gypsum, I have generally called the gypseous formation was deposited, and feldspathic lavas, together with other singular volcanic rocks, were occasionally poured forth: I am far from pretending that any distinct line of demarcation can be drawn between this formation and the underlying porphyries and porphyritic conglomerate, but in a mass of such great thickness, and between beds of such widely different mineralogical nature, some division was necessary.
In those days the rocks which are now the very bones and sinews of our mother earth her granites, her porphyries, her basalts, her syenites were melted into a liquid mass.
Sometimes both varieties meet in the same fragment, as we observed also in the trappean porphyries of the valley of Mexico.
I have been led to this conclusion, from having so frequently observed that, where part of an unstratified mountain-mass resembled in mineralogical character the adjoining streams or strata, there were several other kinds of intrusive porphyries and andesitic rocks injected into the same point.
The included pebbles are well-rounded, varying from the size of an egg to that of a cricket-ball, with a few larger; and they consist chiefly of porphyries. The basis of the conglomerate, as well as some of the alternating thin beds, are formed of a red, rather harsh, easily fusible sandstone, with crystalline calcareous particles.
In those days the rocks which are now the very bones and sinews of our mother Earth her granites, her porphyries, her basalts, her syenites were melted into a liquid mass.
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