132 THE PHYSICAL GEOGRAPHY OF THE SEA. 



fathoms (four thousand and twenty feet), which, being four times 

 Salter than common sea water, left, as we have just seen (§ 252), 

 no doubt in the mind of Dr. Wollaston 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 Mediterra- 

 nean does not pass out again by the Straits ; for it appears by Cap- 

 tain 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. t It is therefore evident, that if water sinks 

 in certain parts of the Mediterranean, in consequence of the in- 

 crease of its specific gravity, to greater depths than two hundred 

 and twenty fathoms, it can never 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.''^ 



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

 and the valleys at the bottom of the sea, especially in the trade- 

 wind region, w^here evaporation is so constant and great, ought to 

 be salting up or filling up w^ith 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 be salt, for the rivers and the rains, 

 it is admitted, bring the salts from the land and empty them into 

 the sea. It is also admitted that the great lakes would, from this 

 cause, be salt, if they had no sea drainage. The Niagara River 

 passes these river salts from the upper lakes into Ontario, and the 

 St. Lawrence conveys them thence to the sea. Now the basins 

 or bottoms of all these upper lakes are far below the top of the 

 rock over which the Niagara pitches its flood. And, were the 

 position assumed by this writer correct, viz., that if the water in 



* *' The Mediterranean." t One hundred and sixty, Smyth. 



X Lyell's Principles of Geology, p. 334-5, ninth edition. London, 1853. 



