30 ON CHANGES IN THE DISPOSITION 



those of the torrent when at a higher level or in a 

 different situation. 



The last source of currents of which I am aware, is 

 that casual and sadden drainage of a lake already no- 

 ticed. It cannot easily occur more than once in any 

 place, though Glen Roy is a noted instance to the con- 

 trary, while its effect may be what is termed diluvian, 

 in every respect. But as the source and direction of 

 the water are here also known, and as its bulk and 

 velocity may often be estimated, it can seldom be dif- 

 ficult to assign the true cause of any effects occurring 

 in such a vicinity. And if a lake can scarcely be filled 

 by deposition, without being also partially drained by 

 the lowering of its barrier, the results may be compli- 

 cated, while generally depending on the comparative 

 rapidity of the two processes. It has been a favourite 

 speculation, as old as Strabo, that the Black Sea broke 

 loose at the Bosphorus in this manner, and that this 

 event was produced suddenly, by the action of earth- 

 quakes ; but no consequences of such a supposed de- 

 luge have yet been pointed out. If the drainage of the 

 plain of Thessaly, already noticed, was effected by per- 

 severing efforts of the river of Tempe, it could have 

 caused no such results. That Glen Roy produced 

 three successive deluges, can admit of no doubt ; but 

 while I refer to the original paper (Geol. Trans.) to 

 show, at least, that the subject was not neglected, 

 under any view, or any needful preliminary theory, I 

 have never been able to trace any effects which could 

 be attributed to a current or deluge, though the dis- 

 charge of water must have been very considerable. 

 And thence also must I receive with great caution, all 

 reports which attribute deposits of alluvia to such a 

 cause ; while, of those which have been thus assigned, 



