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These beds include subordinate layers of greenish impure clay, soft micaceous and calcareous sandstones, and reddish friable earthy matter with white specks like decomposed crystals of feldspar; they include, also, hard concretions, fragments of shells, lignite, and silicified wood.
In this case, we may conclude that the land whence the pebbles were derived, and on which the now silicified trees once flourished, was formed of granite.
I heard in many places of beds of lignite, some of it fine and glossy, and likewise of silicified wood; near the Tolten the cliffs are low, but they soon rise in height; and the horizontal strata are prolonged, with a nearly level surface, until coming to a more lofty tract between points Rumena and Lavapie.
The high land from which we came, appears at present as a distant range to the south-east. Fine-grained sandstone, with impressions of leaves, was again observed, and a few pieces of silicified wood. A Thysanotus with fine large blossoms now adorns the forest.
Besides the upright stumps above mentioned, the dirt-bed contains the stems of silicified trees laid prostrate. These are partly sunk into the black earth, and partly enveloped by a calcareous slate which covers the dirt-bed.
When I visited Aix, I found the silicified wood of this plant very plentifully dispersed through the white sands in the pits near that city. In one silicified trunk 200 rings of annual growth could be counted. Species of Araucaria like those of Australia are also found. Cycads are extremely rare, and of Monocotyledons there are but few.
Strata of this nature alternate with much black and brown siliceo-calcareous slate, remarkable from the wonderful number of huge embedded logs of silicified wood. This wood, according to Mr. Some of the layers of the black siliceous slate contained irregular angular fragments of imperfect pitchstone, which I believe, as in the Uspallata range, has originated in a metamorphic process.
We now came to blocks of silicified wood lying on the surface; it is so like recent wood, that no one who has not handled it would conceive it to be stone and not wood: the outer surface preserves the grain or woody fibre, the inner is generally silica.
Concluding remarks on the denudation and elevation of the Portillo line. Section by the Cumbre, or Uspallata Pass. Porphyries. Gypseous strata. Section near the Puente del Inca; fossils of. Great subsidence. Intrusive porphyries. Plain of Uspallata. Section of the Uspallata chain. Structure and nature of the strata. Silicified vertical trees. Great subsidence. Granitic rocks of axis.
Having already referred to the great lateral valley of the Despoblado, I may mention that above the 2,700 feet of red and white sandstone and dark mudstone, there is a vast mass of coarse, hard, red conglomerate, some thousand feet in thickness, which contains much silicified wood, and evidently corresponds with the great upper conglomerate at Las Amolanas: here, however, the conglomerate consists almost exclusively of pebbles of granite, and of disintegrated crystals of reddish feldspar and quartz firmly recemented together.
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