Page:South African Geology - Schwarz - 1912.djvu/174

 the sandstone exists is riddled with volcanic vents, and the material fills many of the vents whose activity stopped at the stage where the chimneys were drilled, but no volcanic ash or lava came up them. The vents are close together, and many are a mile in diameter, so it is evident that a vast amount of material must have been thrown out by the volcanoes in the initial stages. This non-volcanic material torn from the granite walls of the chimneys would issue as fine sand, and the corrosive action of the hot gases in the vent would account for the alteration of the grains on their surfaces.

The Cave Sandstone is occasionally parted with very fine-grained shale bands containing remains of freshwater pond life, Estheria. Dinosaurian remains of the same nature as those that occur in the Red Beds are also found, together with their footprints, showing that the deposit was terrestrial. The footprints at Morija in Basutoland are three-toed, the middle toe being 11 in. in length.

The Drakensberg Lavas. — The lavas that follow the Cave Sandstone are thin flows of basalt piled one on top of the other for thousands of feet; the greatest thickness measured at Ongeluk's Nek is 5000 ft., but possibly a greater thickness exists in the great Montaux-Sources in Natal, which is a cliff of basalt rising to 11,000 ft. The lava is often vesicular, and the vesicles take the form of pipes about the size of a pencil, usually filled in with agate or various zeolites, or again calcite and chlorite.

The vents from which these lavas have been extruded are crowded together along a broad area, probably a system of fissures which continues the Great Rift Valley of the north, and which bends round and reappears in