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I never saw in Europe such fine hyalites as I found in the island of Graciosa, and on the rock of porphyry called el Penol de los Banos, on the bank of the lake of Mexico. Two kinds of sand cover the shore; one is black and basaltic, the other white and quartzose. The quartzose sand contains fragments of feldspar.
While on this subject it may be added that there is no perennial fountain in this part of the country except those that come from beneath the quartzose trap, which constitutes the "filling up" of the ancient valley; and as the water supply seems to rest on the old silurian schists which form its bottom, it is highly probable that Artesian wells would in several places perform the part which these deep cuttings now do.
Flocks of wild swans, geese, and gruyas winged their way up and down the meandering current of the stream. The sun was setting. The mountains were tinged with an amber-coloured light; and the quartzose crystals sparkled on the peaks of the southern sierra. It was a scene of silent beauty. How long, thought I, ere its silence would be broken by the sounds of ravage and ruin!
The sandstone of Cocollar, which sometimes covers the limestone of Cumanacoa, may be considered as variegated sandstone; but it is more probable that in alternating by layers with the limestone of Cumanacoa, it is sometimes thrown to the upper limit of the formation to which it belongs. The zechstein of Europe also contains a very quartzose sandstone.
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.
Thus, for example, where quartzose beds of mica-schist alternate with clay-slate, or where trap-dykes, often causing waterfalls in the courses of torrents, cut through sandstone or slate these and innumerable other common associations of dissimilar stony compounds must give rise to a very unequal amount of erosion and consequently to lake-basins on a small scale.
In common sand, the quartz grains are the most numerous; but this is not a proof that the rocks from which these particles were derived were wholly, or even chiefly, quartzose in character; for, in many composite rocks, as, for example, in the granitic group, the mica, feldspar, and hornblende are more easily decomposed by chemical action, or disintegrated, comminuted, and reduced to an impalpable state by mechanical force, than the quartz.
It contains, as substances accidentally disseminated in the mass, brown iron-ore, spathic iron, even rock-crystal. As subordinate layers it contains numerous strata of carburetted and slaty marl with pyrites; quartzose sandstone, alternating with very thin strata of clayey slate; gypsum with sulphur near Guire in the Golfo Triste on the coast of Paria.
The quartzose sandstone alluded to, derived from the detritus of the neighbouring granite, is absolutely devoid of carbonate of lime, yet it was formed at the distance only of four hundred miles from a sea-bottom now constituting part of France, where the purely calcareous white chalk was forming.
In the Mesa de Paja this rock contains strata of another quartzose sandstone, very fine-grained; more to the south it contains masses of brown iron, and fragments of petrified trees of the monocotyledonous family, but we did not see in it any shells. This clay, dried and hardened in the sun, splits into separate prismatic pieces with five or six sides.
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