film of iron oxide. Gradually this process causes the boulder to become coated with a skin of black iron oxide, so that it comes to look like an iron cannon ball. In breaking open the boulder the central part will be found to be crumbly, because material has been removed from it to supply the outer coating. This weathering of the rocks from the centre and their hardening on the surface is very characteristic of deserts, and leads to peculiar effects in the shales; here the colouring matter is mostly iron, which becomes leached out and concentrated round the edges of the fragments, whereas the centres become bleached and pulverulent.
Weathering in humid regions is caused principally by the water soaking through the rocks; then, instead of the water being drawn to the surface by the heat of the sun, it is drained through the rock by constant accession of fresh moisture, so that the cementing material and the soluble constituents are leached out and the rock as a whole crumbles. Only those minerals which contain no soluble constituents remain unchanged; all those minerals which contain potash, lirne, or soda, practically the felspars and white micas, and all those containing iron or magnesium, that is to say, the augites, hornblendes, and black micas, yield these substances to be dissolved. The water percolates through the minute crevices of the rock, and causes the rock to crumble away, leaving kaolin and talc, which are clayey substances, behind. Quartz, being practically insoluble at the surface of the earth, is always left behind as sand.
Weathering in snow-covered countries is again somewhat different. The alternations of temperature are here again excessive, and the rocks tend to crack; then the water melted from the snow during the warm hours of
