The Biochronic Equation. 667 
been observed, and near Przibram in Bohemia, an in- 
crease of 1 C. for every 69 meters. Inasmuch as 
these two latter records have been made in districts which 
are further removed from local sources of special high 
temperature than the older mines, we must conclude that 
the earth has already cooled down much further than 
was previously thought, and that the period of 20 to 40 
million years arrived at by LORD KELVIN, is by no means 
too high an estimate. 
GEORGE DARWIN calculates that the moon separated 
off from the earth at least 56 million years ago, and 
GEIKIE put as the maximum for the existence of the 
earth's crust, a hundred million years. The general view 
is that the formation of the sea occurred fairly soon, 
geologically speaking, after the formation of the crust, 
and that no great period of time was necessary for a 
cooling of the water, sufficient to render life possible. 
Further data for similar calculations are furnished 
by the action of rivers. These carry certain dissolved 
salts to the sea. From the mean proportion of sodium 
chloride which they contain and from the total volume 
of water which is poured into the sea every year by all 
of them, we can calculate how much the saline contents 
of the sea must increase from this cause. The total 
amount of dissolved salts in the ocean can also be cal- 
culated and we can then estimate the number of years 
necessary for the accumulation of this quantity. From 
these data JOLY calculated the age of rivers to be 90 
million years; but it is highly probable that the con- 
tinents were originally far richer in salt than now, and 
that the rivers have more or less exhausted them, that is 
that they carried more salt to the sea in former times 
than they do now. Applying this qualification, SOLLAS 
