A. EF. Verrill— The Bermuda Islands; Geology. 67 
times above it. The rain water, being lighter, will rest upon the 
sea water and mix with it only very gradually in the pores of the 
rocks. Even in the shallow wells, often dug near the shore for 
cattle, nearly fresh water can be drawn from the surface during the 
ebb tide, though there may be salt water at the bottom. 
The caleareous fresh water, thus arrested by the sea water, will 
therefore deposit much of its calcite in the strata just above the 
level of the salt water. As this level varies with the tide, a con- 
siderable thickness of harder limestone, often four to six feet thick, 
may eventually be formed about at the level of high water mark, if 
the land should remain at a given level for a long period of time. 
This appears to me to have been the mode of induration of many of 
the hard strata of limestone found in various places, just about at 
high tide level, as along the south shore, and of other hard lime- 
stones on the reefs. 
Such hard compact limestones have been called by some writers 
the ‘base-rock,” and some have believed that they represent an 
older formation, underlying the whole island. 
Mr. A. Agassiz, however, considered them as formed from ordi- 
nary xolian limestones, of any age, indurated by the action of the 
sea water and air, and not indicating any particular period. Both 
views are true in part. 
Superficial induration of the kind to which Mr. Agassiz refers is 
common enough, as described above, but it does not convert thick 
strata of limestone over wide areas, and above sea-level, into a com- 
pact marble-like limestone, of very uniform character ; such as we 
find in much of the so-called “base-rock” of the south shores. 
Doubtless hard limestones of various periods have been massed 
together under the name of ‘base-rock,” and the name is therefore 
misleading and better be abandoned. 
Similar hard limestones occur locally at various higher levels, 
often much above the level of the sea, and they have often been 
quarried for building stones. Some of these belong to the earlier or 
“Walsingham formation” and are associated with the ancient red 
clay and extinct land shells. But others are of later origin and are 
only unusually hard and compact portions of the ordinary xolian 
limestones. 
Perhaps the unusual induration of such layers, distinctly above 
sea-level, may be connected with the somewhat variable zone or 
level of underground fresh water in the rocks, for no doubt such a 
zone exists here, as elsewhere, in spite of the porosity of the rocks.* 
* Artesian wells on the higher lands have yielded water in a few cases. 
