ORIGIN OF ISLANDS. 127 



sixteenth century, only four geographical miles in circumference, but 

 still was richly cultivated and populous. At last, in 1634, in one 

 night, the llth of October, a flood passed over the whole island, 

 whereby one thousand three hundred houses, with many churches, 

 were lost, fifty thousand head of cattle and above six thousand men 

 perished. Three small isles alone remain, and they are still further 

 wasting. It may often be remarked that substances thrown into 

 the sea are not carried down at once to its depths, but rejected 

 many times to the shore, in the direction of the tidal currents. This 

 happens especially with all light, small, and easily moved bodies ; 

 but the case is different with the large blocks of stone, which, con- 

 tinually pressing by their weight downwards, are for the most part 

 gradually withdrawn from the base of the cliff, sunk in the beach, 

 and rolled down to the deep. 



Islands. Islands originate in very different ways. Most volcanic 

 islands are, properly speaking, volcanoes, and have been upheaved out 

 of the sea which surrounds them by the same forces as produce the 

 volcanic eruption. Coral islands, or atolls, on the other hand, are 

 rather examples of rock structures built up out of the sea by coral 

 polyps than islands in the ordinary sense of the term. What is 

 an island now, in the last geological age was probably a submarine 

 shoal, and in the next age is likely to be part of a mountain axis. 

 Hence it is that islands often occur in chains, when they are but the 

 peaks of mountains rising from out of the sea. Or, they often ter- 

 minate peninsulas, or lie contiguous to large masses of land, because 

 some low pass is depressed beneath the sea-level, through which 

 the sea flows, and severs the island from the adjacent land. Thus 

 Tierra del Fuego is but a portion of the Andes, which does not run 

 continuous with the rest of the chain, because it has not been up- 

 heaved so high. Even these continental islands, however, may be 

 formed in various manners. It is comparatively rare for them to be 

 the result of simple uplifting or depression of the sea-bed. Usually 

 the rocks have been more or less folded and fractured; not un- 

 frequently channels, deep or shallow, have been scooped in them by 

 tidal action, or by ice, so that the sea has been able to surround the 

 mass of land. The small islands around our own coast afford examples 

 of the ways in which most of the islands of the world have been 

 produced. Perhaps the simplest illustration of an island, which 

 might result from depression on a small scale, is furnished by 

 St. Michael's Mount near Penzance. There, when the tide goes 

 down, a tiny isthmus is seen to connect the mount with the main- 

 land, while at high tide all trace of union disappears. This method 

 of the formation of islands can be studied fully on any map of 

 Britain on which the levels of the high and low land are marked. 

 For by means of the scale of colours or shading used, it may be 

 observed how with varying degrees of depression, the sea would 

 penetrate into the land, breaking it up into islands and islets. 



Isle of Wight. But a more instructive example is furnished by 

 the Isle of Wight. This island has its chief extension from east 



