GEOLOGICAL CONCLUSIONS. 393 
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 calca- 
reous particles only partially cemented together, deposits this 
substance on the first rocks against which it impinges. But 
the most singular circumstance is that, in the course of a 
couple of months, this layer is either abraded or redissolved, 
so that after that period, it entirely disappears. It is curious 
thus to trace the origin of a periodical incrustation, on certain 
isolated rocks, to the motion of the earth with relation to the 
sun; for this determines the atmospheric currents which give 
direction to the swell of the ocean, and this again the arrange- 
ment 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 Ascension, says, ‘‘The lower part of this, from the 
percolation of water containing calcareous matter in solution, 
soon becomes consolidated and is used as a building stone ; 
but some of the layers are too hard for fracture, and when 
struck by the hammer, ring like flint.” 
The surface of hills of drift sand-rock often has small de- 
pressions that are coated with a smooth, solid crust, as al- 
ready explained. 
IX. FORMATION OF DOLOMITE OR MAGNESIAN CARBONATE OF LIME. 
Analyses of the coral limestone of the elevated coral island 
Metia, by Prof. B. Silliman, Jr., have determined the singular 
fact that, although the corals themselves contain very little 
