Aden, on the Red Sea, is the centre of an extensive volcanic district. Indeed, on both shores of the Red Sea there are a few volcanoes that are still active, while in Sinai, and in the districts of the south, there are several extinct craters. But it is in the New World, especially on the Pacific coast of North America, that volcanic activity was especially great during the geological past. There is a district containing volcanic rocks that extends through various parts of western North America, from New Mexico and North California, to Oregon and British Columbia. This district has a width of from eighty to 200 miles, and a length of not quite 800 miles. This great area of nearly 150,000 square miles is covered with great sheets of volcanic rocks except where mountain ranges rise from them, or where the rivers have cut deep valleys through them. In portions of California and New Mexico these plateaus rise to heights of from 8,000 to 10,000 feet, while in parts of Colorado, where they form huge dome-like mountains, they reach a thickness of 14,000 feet. In Oregon the sheet of lava is 2,000 feet thick, and, indeed, in some places, is estimated to have a depth of 7,000 feet. In the opinion of nearly all American geologists these great lava flows in western North America were not of the type known as crater eruptions, but were what are called fissure eruptions. Some of them are believed to have occurred during geological times as early as the Eocene. Prestwich, however, is of the opinion that the eruptions of the past in these portions of the world were not confined to fissure eruptions, but that crater eruptions also occurred; and that it was towards the close of the Tertiary Age that crater eruptions occurred with great lava flows. Indeed, as we have seen, in portions of Utah and the neighborhood the remains of true craters can be found. Besides the above there are evidences of geological volcanoes of still older times. In portions of Deccan, in southern Hindostan, there is an immense plateau formed of trap rock, that extends from east to west for a distance of 400 miles, and from north to south through from 700 to 800 miles. This district, with an area of almost 200,000 square miles, is covered with a vast lava sheet. It was, in the opinion of Prestwich, from whom many of the facts of the geological volcanic eruptions have been obtained, probably still more extensive. The plateau of Deccan rises gradually from the east to the west, where, in some parts of the Ghauts Mountains, it reaches a height of from 4,000 to 5,000 feet. One of the greatest of these prehistoric volcanoes of Scotland was a volcano in the Isle of Mull in the Hebrides. This volcano was probably nearly thirty miles across at its base, and was from 10,000 to 12,000 feet high. It is now only 3,172 feet in height.