2. In the second place, the geographical position of Britain gives it a notable advantage in regard to the manifestations of volcanic energy. Rising from the margin of a great ocean-basin and extending along the edge of a continent, these islands have lain on that critical border-zone of the terrestrial surface, where volcanic action is apt to be most vigorous and continuous. It has long been remarked that volcanoes are generally placed not far from the sea. From the earliest geological periods the site of Britain, even when submerged below the sea, has never lain far from the land which supplied the vast accumulations of sediment that went to form the Palæozoic and later formations, while, on the other hand, it frequently formed part of the land of former geological periods. It was thus most favourably situated as a theatre for both terrestrial and submarine volcanic activity. 3. In the third place, this advantageous geographical position is found to have been attended with an altogether remarkable abundance and persistence of volcanic eruptions. No tract of equal size yet known on the face of the globe furnishes so ample a record of volcanic activity from the earliest geological periods down into Tertiary time. Every degree of energy may be signalized in that record, from colossal eruptions which piled up thousands of feet of rock down to the feeblest discharge of dust and stones. Every known type of volcano is represented--great central cones like Etna or Vesuvius, scattered groups of small cones like the _puys_ of France, and fissure- or dyke-eruptions like those of recent times in Iceland. Moreover, the accurate manner in which the stratigraphy of the country has been established permits each successive era in the long volcanic history to be precisely determined, and allows us to follow the whole progress of that history stage by stage, from the beginning to the end. These characteristics may be instructively represented on a map, such as that which accompanies the present volume (Map I.). The reader will there observe how repeatedly volcanic eruptions have taken place, not merely within the general area of the British Isles, but even within the same limited region of that area. The broad midland valley of Scotland has been especially the theatre for their display. From the early part of the Lower Silurian period, through the ages of the Old Red Sandstone, Carboniferous and Permian systems, hundreds of volcanic vents were active in that region, while in long subsequent time there came the fissure-eruptions of the Tertiary series.