138 A, E. Verrill—The Bermuda Islands; Geology. 
The variations from the present outlines are, in most cases, no 
greater than might be due to slight inaccuracies of the surveyor or 
engraver. Thus the maps confirm the conclusion that changes due 
to erosion are here very slow. 
22. Origin of the Shell-sands. 
Until quite recently most writers called the calcareous sands of 
Bermuda “coral sands.” Nelson was probably the first writer to 
definitely state that the sands are mainly derived from small shells. 
Mr. A. Agassiz considered them as mostly derived directly from the 
disintegration of the wolian limestones of the reefs and cliffs, though 
ultimately derived from shells, corallines, ete. According to our 
studies, the sand and mud of the sounds, bays, and shores are mostly 
shell-sand, whenever the materials are coarse enough to be identitied. 
But in the deeper parts of the harbors and sounds there is a large 
admixture of calcareous mud, so finely divided that its origin can- 
not be determined directly.* As all gradations exist between such 
fine particles and those that can be recognized as fragments of 
minute shells, it is pretty safe to assume that a corresponding per- 
centage of the fine material is also of shell origin. That a consider- 
able part of the sand and mud is, in many places,-the detritus of 
eroded rocks, especially near the shore cliffs, is very evident, but the 
proportion is probably much smaller than Mr. Agassiz supposed. 
Therefore there is reason to believe that the total mass of material is 
increasing, not diminishing as some have supposed. 
We collected large amounts of the bottom materials from numer- 
ous localities for study, with reference to their origin. Among the 
localities were Murray Anchorage, Bailey Bay, Great Sound, Har- 
rington Sound, Castle Harbor, etc., in depths of 1 to 10 fathoms, as 
well as on the shores. 
They were all rather similar, though differing much in fineness, and 
especially in the relative amount of impalpable mud. When the 
fine mud is washed out through fine sieves, the sand-like material 
that remains consists, in nearly every case, mainly of small broken 
shells, together with many entire specimens, living or recently dead. 
More than 50 species of these small shells can often be picked out 
from a single sample of mud, after washing. In most cases the small 
* See also Verrill, Notes on the Geology of Bermuda, Amer. Journ. Science, 
ix, pp. 328-331, figs. 8, 9, 1900, and Moseley, Notes by a Naturalist on the 
Challenger, 1879. 
