MUSEUM OF COMPARATIVE ZOOLOGY. 137 



in his excellent sketch of the Bermudas, calls attention to the discovery 

 by Pourtales of a conglomerate oif the Florida reef, (the Pourtales Pla- 

 teau,) formed by the remnants of the calcareous remains of numerous 

 invertebrates mixed with coral ooze and sand, which has little by little 

 been built up from great depths. He suggests that the foundation of 

 the Bermudas consists of a submarine bank of a similar nature, which 

 has gradually been built up to the level at which coral reefs can flourish, 

 the Bermuda limestone itself having had its origin upon a mountain or 

 a terrestrial fold, which may consist of rocks having a greater or less 

 geological age. He thus accounts in a most natural manner for the 

 existence of the same rock which forms the surface of the Bermudas at 

 the greatest depths which have been excavated in making the dock at 

 that station. The limestone bank once having been built up to the level 

 at which corals will thrive, the floating embryos carried north by the Gulf 

 Stream found a foothold on which they began to grow, and founded the 

 existing active coral reef. The action of the winds on the beach sand 

 very soon formed the elevated ^Eolian rocks, which rise to a height of 

 over two hundred and forty feet, and of which he, Thomson, 1 and 

 Moseley 2 have given such excellent accounts. Jukes 3 had already, in 

 1845, given a similar account of such an zEolian formation at Baines 

 Island, and Dana, in the Geology of the United States Exploring Ex- 

 pedition, had carefully described the formation of the sand drifts solidi- 

 fied into dunes and encrusting layers along the shores of Oahu. 



Thomson, on page 304 of " The Atlantic," has given a graphic ac- 

 ccount of the mode of origin of the Bermudas, when once the weather 

 edge of the reef was raised above the level of the sea, and of the manner 

 in which the Bermudas of the present day have been built up as a bank 

 of blown sand in various stages of consolidation, though Thomson adopts 

 Darwin's theory, that the atoll of the Bermudas is due to the entire 

 disappearance by subsidence of the island round which the reef was 

 originally formed. 



Thomson also gives, on page 309, excellent figures of the stratified 

 iEolian rocks of the Bermudas, and of ^Eolian beds in process of forma- 

 tion, and on the following pages a figure of a so-called sand glacier, or a 



Darwin'sche Senkungstheorie. Verhandl. d. ersten Deutschen Geographen Tage 

 zu Berlin im Jahre 1881. 



1 Thomson, Voyage of the Challenger, " The Atlantic," 1877, Vol. I., Bermudas, 

 p. 420. 



2 Moseley, N. H., Notes of a Naturalist. 



3 Jukes, J. B., Voyage of the Fly. 



