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. WoUaston 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 agaui by the Straits ; for it appears by Cap- 
tain Smyth's soundings, which Dr. WoUaston 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 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, 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 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." f One hundred and sixty, Smyth. 
i Lyeli's Principles of Geology, p. 334-5, ninth edition. London, 1853. 
