§ 387- CURRENTS OF THE SEA. I35 



ter, left, as we have just seen, no doubt in the mind of Dr. WoUas- 

 ton as to the existence of this under current of brine. But the 

 indefatigable admiral, in the course of his celebrated survey of 

 the Mediterranean, discovered that, while inside of the Straits the 

 depth was upward of nine hundred fathoms, yet in the Straits 

 themselves the depth across the shoalest section is not more than 

 one hundred and sixty* fathoms. " Such being the case, we can 

 now prove," exclaims Sir Charles Lyell, ''that the vast amount 

 of salt brought into the Mediterranean does not pass out again by 

 the Straits ; for it appears by Captain Smyth's soundings, which 

 Dr. Wollaston had not seen, that between the Capes of Trafalgar 

 and Spartel, which are twenty -two miles apart, and where the 

 Straits are shallowest, the deepest part, which is on the side of 

 Cape Spartel, is only two hundred and twenty fathoms.f It is 

 therefore evident, that if water sinks in certain parts of the Medi- 

 terranean, in consequence of the increase of its specific gravity, to 

 greater depths than two hundred and twenty fathoms, it can nev- 

 er flow out again into the Atlantic, since it must be stopped by 

 the submarine barrier which crosses the shallowest part of the 

 Straits of Gibraltar. ":j: 



387. According to this reasoning, all the cavities, the hollows. 

 Vertical circulation and thc vallcvs at thc bottom of the sea, especiallv 



in the sea a physical . , , , . . 



necessity. m the tradc-wiud region, where evaporation is so 



constant and great, ought to be salting up or filling up with brine. 

 Is it probable that such a process is actually going on? No. 

 According to this reasoning, the water at the bottom of the great 

 American lakes ought to remain there forever, for the bottom of 

 Erie is far below the barrier which separates this lake from the 

 Falls of Niagara, and so is the bottom of every one of the lakes 

 below the shallows in the straits or rivers that connect them r.s 

 a chain. We may presume that the water at the bottom of every 

 extensive and quiet sheet of water, whether salt or fresh, is at tlio 

 bottom by reason of specific gravity ; but that it does not remain 

 there forever we have abundant proof If so, the Niagara Kivcr 

 would be fed by Lake Erie only from that layer of water which 

 is above the level of the top of the rock at the Falls. Conse- 

 quently, wherever the breadth of that river is no greater than it 



* "The Mediterranean." t One hundred and sixty, Smyth. 



X Lyell's Principles of Geology, p. 33i-5, ninfl; oc^.itinn. London, 1853. 



