MARINE LIMESTONES NOW FORMING. 107 



an inch or two thick, with the shells more or less broken up and 

 cemented into a calcareous paste, while beneath this bed the deposit is 

 a nearly uniform paste, with only a few shells and fragments scattered 

 through it. Thus it is evident that changes obliterating many of the 

 more delicate organisms are rapidly brought about in such a limestone 

 as is now forming on the floor of the Atlantic. Where the deposit 

 becomes impure, as in the Mediterranean, it is yellow. 



In contrast to such a deep ocean deposit is the white granular lime- 

 stone forming the Bermuda Islands. This substance consists of coral 

 sand, often cemented into a rock which can be polished. It is produced 

 entirely by the wind, and may show from wind action only, according 

 to Sir Wyville Thomson, in a short distance, appearances which re- 

 semble all forms of denudation and unconformity, as well as anti- 

 clinal and synclinal folds. These seolian rocks exhibit most regular 

 stratification, and at Elbow Bay there is, what has been termed, a sand 

 glacier, about twenty-five feet thick, which has come in from the 

 beach, filled up a valley, and is steadily progressing inland. This 

 limestone is full of caves, hollowed out by running water, or by the 

 sea. One of these caves, called the Painter's Vale, contains a lake 

 through which stalagmites rise up sometimes in pinnacles, sometimes in 

 fringes, while from the roof innumerable stalactites several yards long 

 descend and taper to points like knitting-needles. 



Coral islands are a well-known example of limestone masses, built 

 up partly by the direct agency of organic growth, and partly by the 

 power of the waves to grind the coral into sand. Mr. Darwin has 

 described the margin of a coral island as largely formed by great 

 masses of the coral Porites, which are irregularly rounded, from four 

 to eight feet broad, and parted from each other by crooked channels 

 about six feet deep. As the coral extends upward it spreads laterally, 

 so that many of the masses terminate upward in broad flat summits, 

 where the corals are dead. Next in importance is the genus Millepora, 

 which grows in thick vertical plates, so intersecting as to form a strong 

 mass, like a honeycomb. In some reefs the brainstone coral, Mean- 

 drina, abounds. Other stony corals live at a depth of a few fathoms, 

 and in the lagoon are thin, branching, and brittle corals of other kinds. 

 The water outside a reef deepens gradually to twenty-five fathoms. At 

 less than ten fathoms the surface is rugged when not covered with sand, 

 but below twenty fathoms, coral sand is always met with. On the 

 margin of the reef are three kinds of nullipores, extending as a fringe 

 about twenty yards wide, and a few feet higher than other parts of the 

 reef. One of these grows in expanded masses, like certain lichens on 

 trees, another has a radiating structure, and is made up of stony joints 

 as thick as a man's finger ; and the third is reticulated, and formed of 

 branches no thicker than crow-quills. These nullipores and the coral 

 sand are cast up and cemented together by the evaporation of the sea- 

 water. The rolled fragments often form islets, and as the channels 

 are filled up, the surface of the reef becomes a smooth hard floor, as 

 though composed of ordinary limestone. It is this flat surface, from 

 one to three hundred yards wide, which the nullipore margins. It is 



