Principal Depressions on the Surface of the Globe. 255 



cally subjected, forming our haughs, carses, and holms ; the only depres- 

 sions remaining permanently as lakes being those near the sources of 

 rivers, where the feeders that supplied them, being inconsiderable in size, 

 brought comparatively little solid matter along with them, rendering the 

 process of filling up infinitely slow. All our lakes, however, are in pro- 

 cess of gradual obliteration, more solid matter being carried into them 

 than finds its way out ; and all that is required is a sufficient lapse of time 

 to accomplish their extinction, when those at the sources of our streams will 

 undergo the transformation into plains and levels which their predecessors 

 along their tracks have already undergone. The depth of many of our 

 lakes is very great indeed, the bottom of their basins being often very far 

 below the level of the sea ; so that, were their supplies of water dimi- 

 nished, or the evaporation from their surfaces increased, we should have 

 examples presented us, wherever this prevailed, parallel to that with the 

 lakes of Asphaltites, Assal, Tiberias, the Caspian Sea, and many others, 

 of a pool of entirely salt water at the bottom of a hollow lower than the 

 level of the sea ; and to this class of hollows only do we give the name of 

 depressions. 



The bottom of Loch Ness, and of some of the other lakes along the line 

 of the Caledonian Canal, are not only below the level of the surface of the 

 German Ocean, but beneath that of its bed anywhere in the line of their 

 axis across to the shores of Norway. 



Were the Straits of Babelmandel closed, the Hed Sea would be all but 

 dried up in a moderate lapse of years, presenting us with a huge chasm, 

 in some places half a mile in depth, with a long, narrow bitter lake, mar- 

 gined with rock-salt at the bottom. 



The following are some of the dimensions of the most notable of our 

 lakes : — 



Names. 



Area. 



Elevation 

 of surface. 



Depth. 



Bottom 

 below Sea. 





Sq. Wiles. 



240 



32,000 



2,225 



50 



185 



140,000 



T eet. 



1,230 



672 



279 



12,846 



*— 329 



—1,312 



—82 



feet. 



1,012 

 932 

 547 

 720 

 165 



1,300 



Feet. 



300 

 268 



494 



2,612 



82 



Superior ,... 





Titicaca 



Tiberias 



Dead Sea 



Caspian Sea 





I shall turn next to the great continental river basins, or valleys of no 

 outlet, where the rivers on all sides flow towards some central lake or 

 lakes, and the whole of their waters are carried oiT by evaporation. 

 These may be classed under two divisions — those above, and those be- 

 neath, the level of the ocean ; and the first we must note of the first class 

 are those of America— the most notable being that of the Great Salt Lake 



* Mrs Somerville says, in a note on these depressions, that the level of 

 Tiberias as given by actual measurement of Symonds is not to be relied upon, 

 as it falls short by above 100 feet of that determined barometrically by three 

 different observers — Berton, Russerger, and Von YYildenbruch, who give the 

 mean at 755 ; the mean assigned to the Dead Sea by the traveller is 1423-5. 

 With great deference to so distinguished an authority as Mrs Somerville, I 

 should certainly prefer the most ordinary levelling over so moderate a distance 

 to the best barometric measurements where there could be no good barometer 

 of reference to fall back upon. The hour of the day might make all this differ- 

 ence — the barometer read at 10 or 4, without a corresponding reading at the 

 same level at exactly the same hour, would give an error of 100 feet. 



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