116 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



cate at least five additional terraces. They say that there are not many 

 places where the thickness of the coral rock is greater than two hundred 

 feet. In some places it appears to reach two hundred and thirty feet, and 

 in one place two hundred and sixty feet. 



Guppy 1 describes the elevated reefs of the Solomon Islands, where 

 extensive 'degradation of the surface has also taken place, as being of 

 veiy moderate thickness. This agrees with the structure and thickness 

 of the elevated reefs of Barbados and of Cuba, as I imagine them to have 

 been. 



Professor Dana says : " The atoll of the Tortugas, and others in the 

 West Indies, are regarded by Mr. Agassiz as having a basement up to 

 the coral growing limit of pelagic limestone or of some other material. 

 It may be so ; but there is as yet no proof of it." 2 Surely the exten- 

 sion of the miocene rooks of the peninsula of Florida south under the 

 district of the belt where coral reefs are found may fairly be assumed. 

 The existence of the modern limestone of the Pourtales Plateau has been 

 proved by dredging to run close to the seaward limit of the coral reefs. 

 The Alacran atoll, as well as all the coral reefs of the Yucatan Plateau, 

 are underlain by a marine basement (not composed of corals), into the 

 extension of which one can penetrate nearly three hundred feet in depth. 

 And, finally, the recent explorations of Professor Hill and myself along 

 the northern coast of Cuba have proved beyond doubt the existence of a 

 miocene basement underlying the elevated coral reefs. 



While not denying that subsidence is necessary to account in many 

 cases for the formation of deep lagoons and deep channels, it must be 

 left to others to prove that these depressions and channels have been cut 

 through the thickness of the coral reefs, and not through that of the 

 basement. It is quite possible to imagine lagoons to be formed here- 

 after in some of the Bahamas and in the Bermudas by subsidence, which 

 might have a depth of nearly sixty fathoms and yet have a thickness of 

 corals of not more than a few feet on the upper margin, extending along 

 the inner and outer slopes to a depth of not more than twelve or fifteen 

 fathoms. 



The vertical distance between the lower terraces near Saboney was far 

 less than between those observed at Caleta Point. It is quite possible 

 that at the eastern extremity of the island the terraces which are still 

 so plainly marked are not the only terraces once existing. The erosion 

 which made, as we have seen, such great changes in the physical aspect 



1 Tlie Solomon Islands, by H. P. Guppy. London, 1887. 



2 Corals and Coral Islands, p. 292. 



