At the close of the Silurian there was a period of mountain-building and the formation of continents. The extinction of numerous species of marine organisms and the rapid evolution of others point to the seas becoming cooler and the stress of life more acute. In the succeeding Devonian period there is evidence of glacial conditions in South Africa in the form of a thick series of quartzites with striated pebbles up to fifteen inches long, but no typical boulder-clay has been discovered. There are also some doubtful traces from England. The most noteworthy development of the Devonian in the British Isles is, however, a thick deposit of red sandstone (Old Red Sandstone) of the type that is formed in shallow lagoons or inclosed basins, and suggesting desert conditions, so that the rainfall of the British Isles was probably slight. These continental conditions passed away towards the close of the Devonian period, and once again extensive warm oceans appear to have spread over a large part of the globe, associated with the development of reef-building corals. Climate continued warm and equable throughout the greater part of the Carboniferous. The important feature of this period is the great system of coal-beds which extends through North America and Europe to China, with northern and southern limits in 80° N. (north-east Greenland and Spitzbergen) and 15° S. (Zambesi River). Wegener, summing up the evidence, and considering especially the absence of annual rings in the trees, concludes that the coal-beds are the remains of the tropical rain-forest when the equator lay across Europe some 30 degrees north of its present position.