CHAP. IV.] ST. THOMAS TO BERMUDAS. B01 
who was in the habit of collecting corals and knew the local- 
ities of the different species, and under his guidance we ex- 
plored some low caves under the cliffs along the northern shore. 
The roofs of the caves were covered with green algz, and be- 
low the scarcely perceptible line where the air met the still, 
clear water, there was a complete incrustation of the delicate 
coral Mycedium fragile, standing out in undulating purple 
crescents like some luxuriant lichen. Some of the sailors 
stripped and dived for it, and soon there was a pile of beau- 
tiful specimens in the boat. A colony of tropic birds (Phaéton 
wthereus) were building in the cliffs above, and as they sailed 
over us, their two long, white tail-feathers gleaming in the sun, 
their white breasts reflected the color of the water, and they 
looked as if they had been moulded in pale-green glass. We 
rowed slowly round, in full enjoyment, to the corner of the 
Sound nearest Walsingham, and a few minutes’ walk brought 
us to the caves. 
As in all limestone districts, the caves at Bermudas consist 
of large vaulted chambers hollowed out in the rock by the re- 
moval of its material by running fresh water or by the action 
of the sea. The process is probably more rapid in a coral isl- 
and than it is where the rock belongs to one of the older for- 
mations. Dana observed similar caverns in Metia or Aurora 
Island, one of the western Paumotus, in which the geological 
structure may greatly resemble that of Bermudas; and he quotes 
from the Rev. Mr. Williams an account of a cavern in the cor- 
al rock of Atiu, one of the Hervey group, in which he “ wan- 
dered two hours without finding a termination to its windings, 
passing through chambers with fretwork ceilings of stalagmite, 
and stalactite columns which sparkled brilliantly with the re- 
flected torch-light.”* The entrances are usually small crevices 
* “United States Exploring Expedition,” vol. x., GEoLogy, by James D. Dana, 
A.M., Geologist to the Expedition, p. 67. 
I.—20 
