616 . 
The lowest and oldest part of this cap of limestone is of late- 
eocene or early oligocene age. Pirsson’), who examined the boring 
material, was able to prove that in eocene time Bermuda Island 
projected from the sea as a volcanic island and that, probably after 
the voleanie activity had subsided, it was wholly or partially trun- 
cated by wave-action. During that process and afterwards the island 
gradually sank to a depth of somewhat more than 75 metres 
below sealevel and it was during this period of subsidence that 
the reef-limestone was built up on the top of the sinking island. 
The limestone probably always reached about to the surface of 
the sea and hence grew thicker at the same rate as the island 
subsided. 
The fact that in some hills the Bermuda reef-limestone is now 
situated over 40 m. above the sea, proves that the subsidence did 
not proceed continually without true or apparent opposite movements °*). 
The main movement, however continued to be downward. It first 
made itself felt probably in pre-eocene times-and has very likely 
amounted to some hundreds of metres in total. 
A part of this movement, namely the portion that took place 
since the pleistocene period, has probably been caused by a rise 
of the sea-level as explained in Daty’s “glacial-control theory”, but 
not the entire subsidence can be explained in this way. First, it is 
too great considering that the island lies at 35° N. lat., but more- 
over the movement had already begun in oligocene time, as Pirsson 
shows. Besides in the same borehole at 290 m. (935 feet) below 
sea-level, volcanic material was met with again which proved to 
be well rounded by water and showed traces of subaerial weathering, 
which. fact, even if it were supposed that here a part of a sub- 
marine gravel cone was struck, leads one to suppose that the island 
has subsided considerably more than 75 metres. 
This borehole has revealed the fact that where nowadays Bermuda 
island rises from the ocean, submarine volcanoes once were active, 
of which finally one or more rose above the sea, that these volcanic 
islands gradually sank away and disappeared under the ocean-level 
since a period preceding the eocene and that their subsidence for 
a long time has been compensated by the accumulation of successive 
streams of basaltic lava. The layer struck in the borehole at a 
DL. V. Pirsson. Geology of Bermuda island. Amer. Journ. of Science. XXXVIII, 
p. 189, 1914. 
2) Hemprin estimates the average amount of the upheaval preceding the present 
condition of Bermuda Island at 80 feet at least. A, Hemprin, The Bermuda-islands. 
Philadelphia 1889. p. 46, 
