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Updated: May 10, 2025
CALCAREOUS TUFA. Arkose, or granitic grit, with modern shells covered by calcareous tufa. Sea. The heights are given as an approximation obtained from observing the boiling point of water, they are drawn on a scale of 1/10 of an inch per 1000 feet in altitude.
The waste swept in by streams contains much feldspar and other minerals softer and more soluble than quartz, and where the waves have little opportunity to wear and winnow it, it comes to rest in beds of sandstone in which grains of feldspar and other soft minerals are abundant. Such feldspathic sandstones are known as ARKOSE.
Early stages of metamorphism are seen in SLATE. Pressure has hardened the marine muds, the arkose, or the volcanic ash from which slates are derived, and has caused them to cleave by the rearrangement of their particles. Under somewhat greater pressure, slate becomes PHYLLITE, a clay slate whose cleavage surfaces are lustrous with flat-lying mica flakes.
Perhaps now and then the basins became long, brackish estuaries, whose low shores were swept by the incoming tide and were in turn left bare at its retreat to receive the rain prints of passing showers and the tracks of the troops of reptiles which inhabited these valleys. The Triassic rocks are mainly red sandstones, often feldspathic, or arkose, with some conglomerates and shales.
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