It contains a large proportion of mica, a small proportion of quartz and still less feldspar, but generally an abundance of iron pyrites in very minute crystals, which, on exposure, are decomposed.
It was made of black, glassy lava mixed with bits of feldspar.
The purest clay found in nature is porcelain clay, or kaolin, which results from the decomposition of a rock composed of feldspar and quartz, and it is almost always mixed with quartz.
What little feldspar there was is reduced to quartz and mica, and the quartz pebbles are drawn out into lenses.
This is conspicuous for biotite and amphibole crystals but less so in the strongly altered feldspar crystals.
In these first five zones, some crystals, mainly feldspars, are visible in hand specimen.
The presence of quartz-filled microcracks in feldspar crystals suggests that deformation continued during cooling and crystallization.