32 GEOGKAPHICAL CHANaE. 



the vallej of the Mississippi and the basin of the great American 

 lakes, as well as in many parts of the continents of South Ameri- 

 ca and of Africa ; and it is partly, though by no means entirely, 

 owing to topographical and climatic causes that the blight, which 

 has smitten the fairest and most fertile provinces of Imperial Rome, 

 has spared Britannia, Germania, Pannonia and Mcesia, the com- 

 paratively inhospitable homes of barbarous races, who, in the days 

 of the Csesars, were too little advanced in civilized Hfe to possess 

 either the power or the will to wage that war against the order of 

 nature which seems, hitherto, an almost inseparable condition pre- 

 cedent of high social culture and of great progress in fine and 

 mechanical art. 



Destructive changes are most frequent in countries of irregular 

 and mountainous surface, and in chmates where the precipitation 

 is confined chiefly to a single season, and where, of course, the' 

 year is divided into a wet and a dry period, as is the case through- 

 out a great part of the Ottoman empire, and indeed in a large 

 proportion of the whole Mediterranean basin. 



In mountainous countries various causes combine to expose the 

 soil to constant dangers. The rain and snow usually fall in great- 

 er quantity, and with much inequality of distribution ; the snow 

 on the summits accumulates for many months in succession, and 

 then is not unfrequently almost wholly dissolved in a single thaw, 

 so that the entire precipitation of months is in a few hours hur- 

 ried down the flanks of the mountains, and through the ravines 

 that furrow them ; the natural inclination of the surface promotes 

 the swiftness of the gathering currents of diluvial rain and of 

 melting snow, which soon acquire an almost irresistible force and 

 power of removal and transportation ; the soil itself is less com- 

 pact and tenacious than that of the plains, and if the sheltering 

 forest has been destroyed, it is confined by few of the threads and 

 ligaments by which nature had bound it together and attached it 

 to the rocky groundwork. Hence every considerable shower lays 

 bare its roods of rock, and the torrents sent down by the thaws of 

 spring, and by occasional heavy discharges of the summer and 

 autumnal rains, are seas of mud and roUing stones that sometimes 

 lay waste and bury beneath them acres, and even miles, of pas- 

 ture and field and vineyard.* 



* The character of geological formation is an element of very great import- 



