140 ADDENDA. 



On the African coast, there are coral reefs at Port Natal, in latitude 

 30° South ; and here, owing to the warm currents from the Indian 

 Ocean, the sea temperature is not below 68° F., any part of the year. 



P. 115. The following citations from the Journal of C. Darwin, 

 Esq., are important in their bearing on the subject of the consolidation 

 of coral sands : 



In his Journal, p. 588, he says : — " Lieutenant Evans informs me 

 that during the six years he has resided on this Island (Ascension) he 

 has always observed that in the months of October and November, 

 when the sand [of a calcareous beach] commences travelling towards 

 the southwest, the rocks which are situated at the end of the long beach 

 become coated by a white, thick, and very hard calcareous layer. I 

 saw portions of this remarkable deposit, which had been protected by 

 an accumulation of sand. In the year 1831 it was much thicker than 

 during any other period. It would appear that the water charged with 

 calcareous matter, by the disturbance of a vast mass of calcareous 

 particles only partially cemented together, deposits this substance on 

 the first rocks against which it impinges. But the most singular cir- 

 cumstance is, that in the course of a couple of months, this layer is 

 either abraded or redissolved, so that after that period, it entirely dis- 

 appears. It is curious thus to trace the origin of a periodical incrusta- 

 tion, on certain isolated rocks, to the motion of the earth with relation 

 to the sun ; for this determines the atmospheric currents which give di- 

 rection to the swell of the ocean, and this again the arrangement of 

 the sea-beach, and this again the quantity of calcareous matter held in 

 solution by the waters of the neighboring sea." 



Mr. Darwin, speaking of a large beach of calcareous sand composed 

 of comminuted and rounded fragments of shells and corals at Ascen- 

 sion, says, " The lower part of this, from the percolation of water con- 

 taining calcareous matter in solution, soon becomes consolidated and is 

 used as a building stone ; but some of the layers are too hard for frac- 

 ture, and when struck by the hammer, ring like flint." — Journal, p. 587. 



See further on the subject of consolidation, a paper by Prof. Hors- 

 ford in the American Journal of Science, [2], xiv, 245, and another by 

 the author, ibid, xiv, 410. 



