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. 

 Eaisin 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 to 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 cai'bonates to be deposited upon the beaches. 



1 Geol. Mag., March, 1902, IX., p. 132. 



2 A. J. Jukes-Browne. The student's handbook of physical geology, p. 213. 

 London, 1884. 



