worms in South Africa are agents of erosion as well as agents of weathering.
Weathering that does not result in the crumbling of rocks, but, on the other hand, causes them to harden, is very common in South Africa. In shell sand the water penetrates the outer layers and dissolves a certain amount of calcareous matter, which is deposited when the water evaporates, so cementing the grains and producing a fairly hard rock. Ordinary quartz sands exposed on tops of hills become hardened in exactly the same way. Only, as it takes longer for water to dissolve silica, the action is slower; but for all that the silicification of Sands on top of the hills is extremely common throughout South Africa. Especially fine examples are exhibited in the plateau sands of Willowmore and Grahamstown. In England the same process went on in the Eocene Period, when the climate was warmer; all that is left now of these hardened sands are the Sarsen stones isolated blocks of this material which have been used in times past by the Druids for building their temples, as at Stonehenge.
Fig. 12. — Snuff box Shale, upper Dwyka shales; weathering from within. The cementing material of the rock becomes concentrated along the joints, leaving the interior of the blocks white and pulverulent
Somewhat similar to these silicified sands are the brilliant white freshwater quartzites that form such conspicuous objects in the Western Province, Malmesbury, &c. These are due to the silicification of sands in the marshes or vleys, and are probably due to the action of the minute siliceous plants (diatoms) which live in such places.
