CORALS AND CORAL REEFS, ; 3 
from each square mile of the area drained. As the area of 
the Atlantic basin amounts to some 26,400,000 square miles, 
these figures mean that there is annually being poured into 
the Atlantic Ocean lime-salts in solution equivalent to 
1,415,600,000 tons of carbonate of lime. It may be of 
interest to mention that, if this latter quantity of carbonate 
of lime be distributed in a solid form over an area equal to 
that of the land surface from which it was derived, it would 
accumulate, on the above computation, at such a rate that 
one foot of limestone would be formed in about 42,000 years. 
If the total quantity transported seawards annually by all 
the rivers of the globe were to be deposited uniformly over 
the entire ocean floor, it would require 133,000 years for 
the accumulation of a single foot of limestone. This, how- 
ever, is not what really happens, for there are large areas of 
the ocean bottom where almost no carbonate of lime is being 
deposited at all, and where, consequently, the material not 
so used goes to increase the rate at which the carbonate of 
lime may be deposited in other parts. A few other figures 
bearing upon this point may advantageously be given here. 
The total quantity of calcium present in one cubic mile of 
ordinary sea-water is 1,941,000 tons, and that in solution in 
the whole ocean 628,340,000,000,000 tons. Of the quantity 
of lime-salts present in each cubic mile of sea-water, 171,600 
tons consist of carbonate of lime, and the remaining 
1,769,400 tons are of sulphate of lime. If we compare 
the proportions these two salts in a cubic mile of sea-water 
bear to those in the same quantity of river-water, the differ- 
ence is very striking and suggestive. Roughly, the propor- 
tion referred to may be stated as 1 of carbonate of lime to 
17 of sulphate of lime in sea-water, while the average found 
in the river-waters of Europe, according to Mr Mellard 
Reade, shows that the proportion of carbonate of lime to sul- 
phate of lime is as 9°32 to 1:79. Furthermore, river-water 
contains 1 of magnesium to 3 of calcium; while in sea- 
water this proportion is reversed —3°85 of magnesium in 
sea-water being accompanied by 1 of calcium. These 
facts and figures are of much importance in the present 
connection. 
A curious chemical change arises along the zones where 
