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 222 relieve completely and continually the subterranean pressure, so as to free from earthquakes a considerable region round their bases. Thus, while earthquakes agitate the neighbouring islands of the Canary group, the Peak of Teyde appears to be the cause of the comparative immunity from these disasters which the island of Teneriffe enjoys.

By combining observations of this nature with considerations of the grouping of volcanic craters, the direction and extent of earthquakes, the ebullition of hot springs, and analogous phenomena, we arrive at the notion of volcanic regions, and may by this means class the active and extinct volcanos, which are scattered over the globe into a modern number of systems, convenient for description, even if the association rests on an uncertain basis.

In Europe, the purely volcanic phenomena which are from time to time manifested, appear at points referrible to one of seven centres of action. Iceland, with its Geysers and six or more active volcanos, Hecla, Skaptaa Jokul, Skaptaa Syssel, Eyafialla Jokul, and Kattlagiaa, and its formidable eruptions and coast elevations, stands almost alone; Jan Mayen, visited by Scoresby, being its only volcanic neighbour. The Azores form another region of considerable importance, where the rising of islands has happened within the reach of history. Sicily, like Iceland, has, besides the great chimney of Etna, lateral escapes of the imprisoned forces labouring below; and Sciacca, the island which rose and disappeared in 1 831, is a remarkable proof of their energy. The Lipari Isles form another group, where Stromboli and Vulcano are still feebly active; while Vesuvius stands and burns amidst many older vents, long since extinguished, and Ischia and the Ponza isles bear to it the same relation that the Lipari group does to Etna.

The last centre of activity in European volcanic systems is in the Greek Archipelago, where Santorini has undergone many violent displacements, and some igneous exhibitions since the Christian æra.