A. E. Verrill—The Bermuda Islands; Geology. 101 
small. In many cases their formation is partly a mechanical process 
of erosion, but in nearly all cases observed here it is partly or largely 
a solvent action (see pp. 70, 84). 
Eventually the roofs of the broader caverns become too weak to 
support their own weight and they fall in, thus forming “sinks,” 
which, by subsequent enlargement and blending together by erosion 
may, in some cases, form the basins of large ponds, marshes, harbors 
‘and sounds. 
Slow rate of decay of limestones. 
Some data that I obtained by examining the ruins of the old stone 
forts on Castle Island and other islands, some of which were prob- 
ably built before 1620, show an unexpectedly slow rate of disintegra- 
tion of the ordinary firm limestone used in the walls and buildings. 
This was confirmed by observations made at other places. These 
data would make the average rate of subaerial disintegration for the 
harder xolian limestones to be less than two inches in a century. 
This would require 120,000 years for the destruction of the 200 feet 
of hard limestone necessary to form one foot of soil. 
But there are, in many places, areas of much softer limestones, 
which decay far more rapidly and furnish soil much- more freely. 
Such tracts of soft limestones have, by their decay and solution, 
given origin, in many cases, to the sinks, ponds, marshes, and caverns 
that abound on the larger islands. This consideration would very 
materially reduce the time required to form the soil. 
But many of the softer limestones, when exposed to the air, as in 
the road cuttings, become, in a few months, very much harder and 
more resistant to decay. It is rare to find in the extensive road-cuts 
any great portions of the nearly perpendicular side-walls that have 
fallen away by decay. On the contrary, their surfaces have become 
hardened by infiltrations and coatings of calcium carbonate, so as to 
resist weathering quite well. 
My observations, therefore, on this point, though not satisfactory 
and far too few in number, point to a great antiquity for the Ber- 
muda limestones, though recent in a geological sense. 
Spanish Rock. 
Some idea of the slowness of the subaerial decay of the limestone, 
where it has acquired a hard surface, may be gained from an ancient 
incised inscription on the rock known as “Spanish Rock” (fig. 13). 
