Page:South African Geology - Schwarz - 1912.djvu/123

 out at right angles to the pressure as a lump of dough does when one pushes one's fist into it. Static pressure occurs when the rocks are able to sustain the load like a liquid enclosed in a water-tight compartment; no movement occurs but intense alteration occurs in the rocks under the sustained pressure.

There are three zones into which we can divide the metamorphic rocks. In the first, mechanical crushing and shearing is most marked. To this zone belong the quartzites, phyllites, chlorite- and talc-schists, and the schists with the lustrous white mica, sericite. In the second zone the crystals formed occupy a smaller space than the substance of which they are composed did in the original rock. To this zone belong the quartzites, marbles, the true biotite and muscovite schists, and a few of the gneisses, that is, schists with felspar, such as the hornblende gneisses. In the deepest zone the pressure is nearly entirely static, and the rocks approach the igneous rocks in the nature of their crystalline structure; the schistose and banded structures become less marked, until in rocks like the granulites and eclogites it has disappeared altogether. To this zone belong the biotite and augite gneisses and the granulites. Taking mud as the most complex, chemically, of the sediments, we have the following stages:


 * 1) Simple compression ... ... ... Result, shale.
 * 2) Intense compression, some cementing ,, slate.
 * 3) Movement under pressure; thin layers separated by being squeezed out at right angles to pressure; planes of cleavage developed ... .... ... ,, roofing slate.
 * 4) Development of mica flakes along planes of movement ... ... ... ,, phyllite.
 * 5) The whole rock recrystallized between