How the Waters of the Ocean became Salt. 167 
and become supersaturated by evaporation. But the mode of occur- 
rence of the known beds of rock salt lend little support to this 
view; and recent investigations have led to the conclusion that 
deposits of rock salt have been accumulated over the floors of 
inland salt lakes like that of the Dead Sea in Palestine, along 
whose banks such deposits occur in the form of terraces which 
once formed the bed of the inland lake itself, when at a higher 
level than at present, but owing to the lowering of its waters 
are now exposed along its western margin, as in the case of the 
terraced hill known as Jebel Usdtiim. Another fatal objection to 
the view of the marine origin of salt-rock is to be found in the 
fact that this rock generally consists of nearly pure chloride of 
sodium, while ocean water contains large proportions of the 
chlorides of calcium, magnesium, and potassium, the precipitation 
of which would result in a deposit very different from that of the 
rock-salt of Cheshire and Worcestershire, which is composed of 
98°30 per cent. of chloride of sodium and only small traces of 
other salts. 
7. But in addition to the evidence derived from organic forms 
of the primeval ocean we apparently possess very remarkable 
direct evidence that the waters were highly saline. It is known 
that some strata of the Upper Silurian period in North America 
are saliferous, constituting the Onondaga salt group and the Trenton 
and Chazy limestone series! These strata are characterised by 
large numbers of marine organisms, and there can be no doubt 
that they were formed in the waters of the Silurian seas. They 
also yield large quantities of saline waters which are used in 
commerce, and in which chloride of sodium predominates; and as 
the strata are often in the condition of basins below the level of 
the outer ocean, Dr. Sterry Hunt has inferred that the waters with 
which they are saturated were originally those of the Palaeozoic 
ocean in which the strata were deposited. In other cases, however, 
where the strata are upraised above the ocean level and highly 
inclined, the same author considers that surface waters have 
gradually replaced those originally contained in the strata? Thus 
we are justified in inferring, not only from organic, but from 
direct physical evidence, that the waters of the early Silurian 
oceans were salt. 
8. On examining samples of water taken from the open ocean of 
various regions and far from land, it has been found that the propor- 
tion of salts and carbonates do not vary much. This is doubtless 
owing to that wonderful system of currents by which the waters are 
kept in a state of perpetual movement, and owing to which there is 
a constant interchange of the warmer waters of the equatorial region 
with the colder of the polar. Sea water is essentially a chlorinated 
1 Dana states that in the State of New York the salt is made from strong brine by 
sinking wells varying from 150 to 340 feet in depth. It takes from 35 to 45 gallons 
of this water to make a bushel of salt, whereas it takes 350 gallons of sea water for 
the same result. 
* «<Chemical and Geological Hssays,’’ p. 104. 
