STRUCTURE OF THE TURKS ISLANDS 263 



exposed between the tide-marks on the sandy islets of coral-reef 

 regions all round the tropics, and forms the basement of the islet. 

 Upon this foundation are heaped up by the waves, as is described 

 by L. Agassiz, masses of sand, broken shells, and fragments of 

 corals, which in their turn are covered by the finer sand driven by 

 the wind and forming sand-dunes. He does not refer to any ex- 

 posure of an actual contact of the aeolian rock with its foundations ; 

 but we may infer that they would be the materials heaped up by the 

 waves, but more or less consolidated and cemented, and passing 

 downward below the high-water level into the basement rock, 

 similarly composed of sand, broken shells, and coral debris, that 

 constitutes the foundation of the islands. 



It was only in one locality in the Turks Islands that I found a 

 good contact-exposure of the rocks underlying the aeolian sandstone. 

 This was on the east coast of Greater Sand Cay near its southern 

 end. Here, about two or three feet above the high-water level, the 

 aeolian rock sometimes passed down into a coral-rag, composed of 

 large fragments of massive corals imbedded in a matrix of compacted 

 coarse sand, and at other times into an ordinary coarse beach sand- 

 rock such as is described below. 



" Beach sand-rock " came under my notice in different islands of 

 the Turks Group, as on Grand Turk and Salt Cay. It is character- 

 istic of low coral islands all over the tropical Pacific, and doubtless 

 is common enough in the Bahamas. It is well described by Dana 

 in his Corals and Coral Islands (1872, pp. 152, 184), and I have dealt 

 with it in my Geology of the Solomon Islands (1887, p. 84). It is a very 

 coarse white sand-rock, and is stratified, the beds dipping seaward 

 with the slope of the beach at an angle usually of seven or eight 

 degrees. It is composed of coral and shell debris; but many 

 characters distinguish it from the aeolian sandstone or drift sand-rock, 

 notably the larger proportion of coral debris, the coarseness of the 

 constituents, and their great irregularity in size. Sometimes the 

 materials forming the beach sand-rock are imperfectly consolidated. 

 At others they are firmly cemented into a hard rock with a metallic 

 ring ; and when, as is not infrequent, large fragments of corals and 

 dead shells are enclosed in the hard matrix it might be termed a 

 coral-rag, though reef-rock would perhaps be a more appropriate 

 name. The typical rock is exposed between the tide-marks. But 

 it is a formation that is only exposed by the removal of the overlying 

 loose sands by the waves. Wherever exposed, it is always in process 

 of destruction through the action of the sea. I have never seen it 

 in process of formation. The conditions under which the stratifica- 

 tion and consolidation take place are obscure. Evidently, whether 

 as sand-rock or reef-rock, it forms the platform on which islands and 

 islets are thrown up by the waves; and it would be on such a 

 foundation that the subsequently produced aeolian deposits would 

 be based. 



The upshot of the foregoing remarks would appear to be this. If 

 there has been an extensive subsidence of 300 feet in the Bahamian 

 region since the formation of the aeolian sandstone, as is assumed by 

 A. Agassiz, the foundations of the aeolian rock ought to lie far beneath 



