But it is not only on the surface of the land that volcanic eruptions take place; for in some cases the outbreak of a submarine eruption has been witnessed, and it is highly probable that in past geological ages many large eruptions of this nature have taken place. In the year 1783, an eruption took place about thirty miles off the west coast of Iceland. An island was built up from which glowing vapour and smoke came forth; but in a year or less the waves had washed everything away, leaving only a submerged reef. The island of Santorin, in the Greek Archipelago, is a partly submerged volcano. But in some cases enormous outpourings of lava have taken place, not from volcanoes, but from openings of the ground here and there, and more usually from long fissures or cracks in the rocks lying at the surface. In many cases so much lava has quietly welled out in this way that the old features of the landscape have been completely buried up, and wide plains and plateaux formed over them. Sir A. Geikie says,-- "Some of the most remarkable examples of this type of volcanic structure occur in western North America. Among these that of the Snake River plain in Idaho may be briefly described. "Surrounded on the north and east by lofty mountains, it stretches westward as an apparently boundless desert of sand and bare sheets of black basalt. A few streams descending into the plain from the hills are soon swallowed up and lost. The Snake River, however, flows across it, and has cut out of its lava bed a series of picturesque gorges and rapids. "The extent of country which has been flooded with basalt in this and adjoining regions of Oregon and Washington has not yet been accurately surveyed, but has been estimated to cover a larger area than France and Great Britain combined. Looked at from any point on its surface, one of these lava plains appears as a vast level surface, like that of a lake bottom. This uniformity has been produced either by the lava rolling over a plain or lake bottom, or by the complete effacement of an original, undulating contour of the ground under hundreds of feet of lava in successive sheets. The lava, rolling up to the base of the mountains, has followed the sinuosities of their margin, as the waters of a lake follow its promontories and bays." A few further examples of mud-lavas may be mentioned here. Cotopaxi, a great volcano in Ecuador, South America, with a height of 17,900 feet, reaches so high into the atmosphere that the higher parts are capped with snow. In June, 1877, a great eruption took place, during which the melting of snow and ice gave rise to torrents of mud and water, which rushed down the steep sides of the mountain, so that large blocks of ice were hurried along. The villages around to a distance of about seventy miles were buried under a deposit of mud, mixed with blocks of lava, ashes, pieces of wood, etc.