The continental area in which the Old Red Sandstone lakes lay (a kind of far Western Europe without a Russia) began now to sink in its turn. All of the British Isles, except a very thin slice just cut across the Midlands from North Wales to Norfolk, was sunk beneath the sea. The lakes disappeared, and above their deposits, as above the rest of England and nearly all Europe except Scandinavia and patches of Spain, Italy, and the Balkans, a deep ocean rolled, and for many thousands of years deposited a grey ooze of limestone. This limestone is called the Carboniferous or Mountain Limestone. But as time went on this old sea floor began to be slowly raised, and in the shallower waters a great quantity of coarse sand and stones and conglomerate--the Millstone Grit, as it is called--was deposited. Limestone denotes clear seas; but the borders of clear seas are often the sites of accumulation of land rocks, and the clear waters of the early Carboniferous sea which stretched from Ireland to the north of Europe were bordered by shores along which mud and shale, gravel and sand were deposited. The end of this period was marked in Europe by great disturbances of the earth's crust--though perhaps these disturbances, as we have shown in a previous chapter, were not sudden or violent, but were slow upheavals, lasting hundreds of thousands of years. It was at this time that a great system of mountains, sometimes referred to as the Palæozoic Alps, began to rise. This system of mountains crossed the central part of Europe from the Western Islands to the Sudetes Mountains in the east. Their remnants are seen in the Vosges Mountains, the Hartz Mountains, and the Black Forest at the present time; and the development of the Ural Mountains was contemporaneous with them. During this time a mild climate spread all over Europe, and as far north as Spitzbergen the waters were warm enough to support coral reefs and plants which we associate with the seas of genial latitudes. In time the Carboniferous sea became quite filled up; and its floor was raised up to or a little above the waters. Then in great swamps, marshes, and low lands, the burgeoning vegetable life of the northern hemisphere entered on its long-deferred reign. It was then that the coal which we burn in our grates to-day was laid down. Let us consider the circumstances in which coal is to be found. The coal formations, as we know them, are found in the same state, and evidently laid down in the same era, from the Equator up to Melville Island in the Arctic regions, where in our day it is always freezing. They stretch from Nova Zembla to the middle of China; and they are much the same in New Zealand and New South Wales. Therefore the first conclusion we draw was that nearly all over the globe the climate was the same--hot, close, moist, muggy. Whatever the climate was the growth of vegetation was tremendous.