190 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
over twenty metres high on the eastern side of the Red Sea. In Wadi 
Hashubi he found the sand grains cemented by carbonate of lime, and 
at the mouth of Wadi Nasb, are gravels cemented by calcite. Dr. 
Raisin says there are low raised beaches on Perim Island in the Straits 
of Bab-el-Mandeb.* 
Without more definite information it is impossible to say whether or 
not the beach deposits of the Red Sea are similar in nature and origin 
to those of the coast of Brazil. Unfortunately a letter of inquiry, regard- 
ing this matter, to the geologist in charge of the Egyptian survey 
received no reply. 
Leaving the Red Sea out of account, there still remains the question 
why this hardening is not more general: why is it apparently confined 
to the beaches of the Mediterranean Sea and of northeast Brazil? 
RELATIONS OF DENSITY то DEPOSITION. 
The salt water of the sea of a necessity cannot hold in solution. as 
much lime carbonate as can fresh water. This is due to the fact that 
sea-water already holds so much mineral matter, most of which is more 
soluble than the carbonate of line. It follows for the same reason that 
the denser the sea-water the less carbonate of lime it will be able to 
hold, or the more ready it will be to give up and deposit any that it 
may have or receive in solution. We should, therefore, expect that 
waters holding much carbonate of lime in solution, on flowing into the 
sea, would deposit it more promptly in denser than in less dense sea- 
water. 
Jukes-Browne points out “that where a body of fresh water, containing 
much carbonate of lime in solution, enters the sea, and remains exposed 
to surface evaporation, a precipitation of carbonate of lime will take 
place.” ? 
There is a perceptible and probably constant variation in the density 
of sea-water in spite of its movements and its commingling, and this 
variation must influence the precipitation of lime in the beach deposits. 
Furthermore, any increase of the density of the sea-water would hasten the 
precipitation of the carbonates, and as it is precisely in the tropics and 
in arid regions that the sea-water is densest, it is there that there is the 
greatest tendency of the carbonates to be deposited upon the beaches. 
1 Geol. Mag., March, 1902, ТХ., p. 182. 
2 A. J. Jukes-Browne. The student’s handbook of physical geology, р. 218. 
London, 1884. 
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