The glacial period was not one and simple, but comprised several phases. During the Pliocene the climate was gradually becoming colder, and either towards its close or at the commencement of the Quaternary, this culminated in a first and most intense glaciation. Ice-caps radiating from Scandinavia crept outwards, filling up the North Sea, crossing valleys and mountains, and covering with their boulders and moraines a wide circle, embracing Britain down to the Thames valley, Germany to the Hartz mountains, and Russia almost as far east as the Urals. In North America a still more massive ice-cap overflowed mountain ranges 3000 feet high, and covered the whole eastern half of the continent with an unbroken mantle of ice as far south as New York and Washington. At the same time every great mountain chain and high plateau sent out enormous glaciers, which, in the case of the Alps, filled up the valley of the Rhone and the Lake of Geneva, buried the whole of the lower country of Switzerland under 3000 feet of ice, and left the boulders of its terminal moraine, carried from the Mont Blanc range, at that height on the opposite range of the Jura. Nor is this a solitary instance. We find everywhere traces of enormous glaciers in the Pyrenees and Carpathians, the Atlas and Lebanon, the Taurus and Caucasus, the highlands of Scotland, Ireland, and Wales; in the Rocky Mountains and Sierra Nevada; the Andes and Cordilleras of South America; in South Africa and in New Zealand. These may not have all been simultaneous, but they certainly all belong to the same period of the great glaciation, and show that it must have been affected by some general cause, and not been entirely due to mere local accidents.