Changes of l,evel i» the Bermuda [slaiuh. — Tarr. 297 
In structure the Bermudas are mainly composed of wind- 
blown shell and coral sand, confusedly thrown together in the 
manner common to aeolian deposits. The wind-drift structure 
of the consolidated coral sand is everywhere apparent, in road- 
cuts, in quarries and sea cliffs, as well as upon the outcrops 
in the fields. This is one of the most noticeable features f)f 
the geology of the islands. Not only is the evidence of the 
jeolian origin distinct, but the process of dune formation may 
still be seen along the south shore, where the shell fragments, 
ground to bits on the beach, are moving inland over the sur- 
face of the islands.* 
While the greater portion of the islands which rise above 
the sea, is made of wind-blown sliell sand, it is e(]ually certain 
that in a number of places this rests on a pedestal of denser 
rock, the "base rock," which is older and much moie compact 
than the sand hill rock. This base or beach rock ma}^ be seen 
at various places beneath the cliffs of the south shore, and it 
was also discovered in the boring at Ireland island. Where 
in contact with the sand rock its surface is verj' irregular, f 
but in no place was it seen to a hight greater than fifteen feet 
above the sea. In other places it appears to be absent, al- 
though, as in the case of Ireland island, it may still be pres- 
ent at some distance below the sea level. While consolidated 
layers f)f the blown sand are generally soft and crurabl}'-, the 
beach rock is always dense and hard, ringing under the ham- 
mer, like some even-grained igneous rock. 
Accumulation of the liase Bock. As Rice has so satisfac- 
torily proved, this dense stratum was accumulated under beach 
conditions. Its counterpart in unsolidified form is now to be 
seen on the various beaches of the south shore. The texture 
of the grains composing it is distinctly coarser than that of 
the wind-blown deposits, and in it may be seen innumerable 
fragments of shells and corals which have been ground up by 
*Tiie8e dunes have been described by previous observers, and a pho- 
tograph of a house partly buried beneath the encroaching sand, was 
taken by Thomson, and published in "The Atlantic" (p. 312). The 
view shows the chimney and a part of the roof projecting above the 
sand. At the time of my visit the chimney was nearly covered: 
and a photograph then taken, showed very decided increase in the sand 
accumulation. However, by the growth of oleanders and other trees, 
the sand movement is now nearly checked. 
IThis is well seen in Thomson's section, The Atlantic, vol. i, p. 319. 
