GEOLOGICAL CONCLUSIONS. Ill 



These reef-rocks receive also large contributions of sand or 

 fragments from shells, which unite with the coral debris. 



Besides the coral rocks formed beneath the waters surface, the 

 beach or drift sands and gravel form stratified deposits of consid- 

 erable extent, either bordering the shores within the range of the 

 tides, or raised some distance above them. These drift-sand for- 

 mations may cover the forming island and contribute largely to 

 its progress ; and sometimes they accumulate into drift hills form- 

 ing when consolidated, hills of rock occasionally 60 or 80 feet in 

 height. The rock is often loose and friable, but sometimes quite 

 solid. The layers of the beach rock have a nearly constant dip 

 of a few degrees, not exceeding eight, towards the water. The 

 rock of the drift-hills or accumulations is more finely laminated, 

 less firmly cemented, and dips whichever way the accumulating 

 sandhill sloped, the layers being the successive sheets of sand 

 which were drifted over it. 



II. Coral reefs, though they may stretch along a coast for scores 

 of miles, are seldom a single mile in width at the surface : and if 

 elevated above the sea, they would stand as broad ramparts sepa- 

 rated by passages mostly 20 to 200 feet deep, and often of great 

 width. The substratum, however, is continuous coral-rock ; and 

 if these more elevated parts were removed by any process, after 

 an elevation, they would leave an area of coral limestone often as 

 extensive as the whole reef-grounds. This is at once seen from 

 the figures of the Kingsmill chart and others on a preceding 

 page. In an island like Deans's, one of the Paumotus, these reef- 

 grounds are 1000 square miles in extent. It is true that the reefs 

 at the surface gradually widen if the land is undergoing no sub- 

 sidence. Bat when situated on a sloping bank, as usual, this 

 widening gradually renders the bank steeper, and the rate of 

 increase in width is rapidly diminished. And if the bank were 

 not sloping, there is still reason to believe that the patches would 

 not attain a great width at the surface of the sea, owing to the 

 currents sweeping over them, occasioned partly by the position 

 of a growing reef; and that therefore there would be unoccuoied 

 intervals or channels, as above alluded to, between the several 

 reefs of a reef-ground. 



The bearing of these facts upon the character and origin of 

 ancient limestones, and the formation of channels, or valleys in 

 such rocks, is apparent without particular explanation. 



III. The occurrence of coral sand forming the exposed beaches, 

 while the finer coral mud exists on the shores of the smaller la- 

 goons, or at the bottom of the larger, affords an interesting illus- 

 tration of the result produced by a triturating sea, as compared 

 with that from more gently agitated waters. The rnde seashore 

 waves give rise to sand or pebbles; while the gentle undulations 

 or rippiings of inland waters produce mud by their finer tritura- 



