85 
beneath the water, between the Pedro shoal and the 
open sea; on these the winds and the currents drive 
a heavy surf. The spots properly called the islands 
aré seven in number, and vary from forty to some 
three and four acres in size. ‘They are upthrown 
masses of broken coral and sheli cemented by calca- 
' reous sand, washed upon rocky ledges above the sea. 
The breakers shift with the shifting winds, rolling 
these fragmentary deposits on before them. By the 
regularity of their change of action, they have done 
the work of accumniation pretty equally on all 
sides: they have raised a wal! all around the islands, 
and left the centres hollow. 
From time to time sterms of unusual vialence 
have carried the heaped-up coral and sand suddenly, 
and in thick,layers, over portions of the islands 
where the dung of the sea birds had accumulated for 
years, and these irruptions have made intermediate 
deposits of animal matter and cemented rock. it is 
evident from the prevalence of this succession of 
deposits withia the hollow centres of the islets that 
the sea has washed in the fragmentary materials of 
the outer margins, by a more than ordinary rise of 
the waters, and laid them in pretty equal strata at 
distant intervals of time, so that the centres have 
risen in height as the sea walls have been built and 
cemented up. ‘The animal deposits which may be 
characterized as loosely cohering urate of lime are 
sometimes found two feet beneath the strata of 
cemented coral and shells, and run about an inch 
Immediately within the islands, the water shoals, 
