Much of the earth’s crust lies in a parallel pattern of horizontally banded layers called stratum. Stratum are layered by age, with the youngest deposition of earth on the surface and progressively older layers at increasing depth. Each stratum is composed of a distinct rock or soil, formed by a geological process such as material deposition via ocean tides, river flows, and volcanic eruptions; potentially in combination with the metamorphosis of rock via compression. The stratum and the boundary levels between strata are subject to further transformation from weathering, both the chemical process and the physical process of water (usually rain) carrying new compounds into contact with the rock and carrying other compounds or parts of the rock itself away from the strata. The law of superposition applies to the strata of clays formed by the sedimentary deposits of Yixing, with older and thus (generally) more weathered layers found in deeper strata.
Zisha is a secondary depository siltstone or mudstone compressed into ore through time and weathering; the scattered deposits of zisha ore are embedded into or layered within other larger non-zisha clay strata. Zisha-ore is mined from tabular or stratiform lenses found embedded into or between these non-zisha clay layers of sedimentary rock and clay. The “Yellow Stone Layer” (黄石层, Huang Shi Ceng) is the major non-clay stratum in Yixing, found at multiple depths; the stratum is composed of a high-quartz sandstone of grey-yellow color deposited during the carboniferous period; the porous sandstone allows water percolation through the bed, catching, carrying or filtering clay and minerals, shaping the weathering of the clay-material above and below. Different zisha-ores are found in different rock or clay strata, and many ores are formed by chemical weathering at the boundary-layers between layers, including above and below the Yellow Stone Layer.
To summarize: zisha ore is found incased by or layered with non-zisha clay which modulates is formation and weathering; the non-zisha clay of Yixing is itself a usable ceramics material and was mined as a byproduct of the rarer zisha ore. Knowledge of the non-zisha clay strata is necessary to understand the mining history and historical use pattern of zisha ore for teapots.
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