PHYSICAL GEOGRAPHY. 101 



Many, as hasbeen elsewhere observed, have been evidently 

 the result of volcanic action. It is said that nearly half 

 of the islands are indebted for their origin to this most 

 potent agency. In the time of Seneca, an island called 

 Therasea suddenly emerged from the ^gean Sea. Pliny 

 mentions thirteen as having appeared at the same time. 

 But to confine ourselves to later times : in 175T eighteen 

 small islands suddenly appeared near the Azores, but 

 they at length disappeared. In 1811, a volcanic moun- 

 tain rose out of the sea near St. Michael's, one of the 

 Azores. In 1814, an island of this nature arose among 

 the Aleutian Islands. In 1831, an island suddenly made 

 its appearance near the Lipari Islands, in the Mediterra- 

 nean, when an English vessel was accidentally in sight : 

 the commander of the vessel, Lieut. Graham, landed 

 and took possesion of it in the name of his Britannic 

 Majesty, but after a few months it entirely disappeared. 

 Numerous islands are the work of the Coral insect; 

 while some are formed by the action of water, separating 

 a portion of land from the main hind. Great Britain 

 may have been thus formed. Islands are also 

 often formed by sand banks in the mouths of rivers. 

 Some of those formed at the mouth of the Ganges, of 

 which there are many, are more than thirty miles in cir- 

 cumference. 



^SPRINGS AND RIVERS. 



Springs and rivers constitute a peculiar feature of the 

 Earth's surface. Springs derive their origin from the 

 condensation of aqueous vapour, which, entering the 

 ground in the form of rain or mist, continues to sink 

 until it has arrived at some rock impermeable to water, 

 when it finds means to gush out sometimes in a very 

 considerable stream. The cavernous parts of mountains 

 will occasionally become reservoirs for the collection of 

 water, which, flowing out of their sides, will form the 

 source of rivers. The largest rivers of Europe, Asia, and 

 America, take their rise in the higher mountains. Thus 

 the Danube, the Rhine, and the Rhone take their rise 

 from the Alps ; the Ganges, the Indus, and the Brah- 

 mapootra, from the Himalayan mountains j and the 



