Page:South African Geology - Schwarz - 1912.djvu/128

 gangue. The minerals may be washed down by rivers and collect in alluvial patches, where by washing the alluvial gravels the minerals may be recovered. The most frequent application of this method is in gold washing; gold occurring in river gravels is called placer gold. Tin is commonly met with under the same circumstance, and such alluvial tin is called stream tin. The bankets of the Rand are old river gravels hardened and compacted, and the patchy nature of the original placer gold averaged by water circulating in the matrix when the rock was deeply buried, and the water, being under great pressure, acted as a powerful solvent.

SECTION IV

STRATIGRAPHICAL GEOLOGY

On the corner of a building in Grahamstown the whitewash had peeled oft, and there stood revealed a name painted on the wall which had long been covered. There were two layers or strata of material on the wall; the older, with its peculiar characteristics, became exposed by the removal of the younger. So in the innumerable strata of the rocks, the older are revealed on the surface by the younger being removed by denudation, and each layer, having its own peculiarities, can be recognized in widely separated regions. The order of the superposition of the rocks is always the same; just as in the illustration of the house the more recently applied whitewash could never be found under the older coating bearing the inscription, so the more recently deposited sediments can never be found under the older ones.