Page:South African Geology - Schwarz - 1912.djvu/124

 planes of movement, which become planes of schistosity ... ... ... Result, schist.

6. Felspar introduced from outside ob- literating planes of schistosity; rock banded with layers of various mine- rals; foliation planes ... ... ... ,, gneiss. 7. Banding and all traces of shearing dis- appear ,, granite.

Sandstones, being more or less rocks containing only the mineral quartz, can only alter into quartzite, but if there is some shaly matrix with the sand grains the sandstone may become a quartz schist.

Limestone alters to marble, but under great pressure its solubility causes it to simply disappear. Thus, in the Kheis rocks of Prieska the limestone bands only occur in places where the pressure has been comparatively small; when they are followed into the areas of greater compression they taper out. The lime frequently goes in solution into the surrounding rocks and causes the development of garnets and other lime silicates.

Contact Metamorphism. — When a tongue of liquid rock intrudes into a sedimentary rock, one would expect to find that the latter was altered at the contact. This does occur; thus, in the coal mines of Indwe and Cyphergat, dykes of dolerite pass through the coal seams and a yard or two of the seam next the dolerite is anthracitized, that is, has all the gaseous content removed and is in the condition of coke. In most of the dolerite contacts the alteration is quite insignificant; the shales are occasionally bleached and hardened, at other places nodules of chlorite and white zeolites appear in the slate near the contact. Real metamorphic mineralization occurs in the Free State, where olive-brown vesuvianite and brown garnet are sometimes found in the slate near the dolerite.