68 A, E. Verrilli— The Bermuda Islands; Geology. 
Percolating calcareous waters would be arrested at such a level, 
and deposit, just above it, by cooling and evaporation, much calcare- 
ous material, for it would occupy a zone in which evaporation would 
be taking place in dry weather and through capillary action. This 
process, if long continued, would form strata of hard limestone, but 
not necessarily of any particular period. 
The harder strata, especially of the “ Walsingham formation,” are 
usually overlaid by a bed of red-clay soil, which was probably orig- 
nally occupied by vegetation. The dead vegetable matter of such 
a soil may have contributed additional carbonic acid, and perhaps 
humie acids, to the percolating rain water, and this may have has- 
tened the solidification of the underlying rocks. 
Therefore it is evident that no very definite separation of these 
limestones into periods or formations can be safely made, merely on 
the hardness or compactness of the rocks, though in general the 
older ones are likely to be the harder. The sands have been con- 
tinually drifting and consolidating, unequally and variously, ever 
since the first islands rose above the sea, and at all levels. So, like- 
wise, the changes in elevation and subsidence have been so very 
gradual that they have produced no marked periods or changes in 
the rock formation, except locally. We can, however, distinguish 
an earlier period, by means of the extinct fossil land shells, during 
which the climate was more favorable for vegetation and land-snails 
than at present, as indicated above. 
This we may provisionally refer to the Pliocene. It was, in my 
opinion, certainly pre-glacial. 
14. Pliocene Bermuda; Walsingham Formation. 
I propose the new name “‘ Walsingham formation” to designate 
that portion of the older Bermudian strata of limestone and red clay 
characterized by containing several species of extinct land snails, of 
which the largest and most abundant is the Pectlozonites Nelsoni 
(Nelson’s snail ; pl. xxvi, figs. 5-8). 
a. Compact Limestones. 
The most prominent and characteristic of the rocks are the com- 
pact and hard eolian limestones which have, in many places and 
over wide areas, become so highly infiltrated with calcite, that the 
original sand-drift structure has been obscured or lost, so that they 
sometimes appear to be in thick massive strata, forming durable 
