10S ON CORAL REEFS AND ISLANDS. 



On the American shores of the Atlantic there are few reefs, 

 except in the West Indies. The waters of the Orinoco and 

 Amazon, and the alluvial shores they occasion, exclude corals 

 from that part of the coast. But about Pernambuco, as I am in- 

 formed by Mr. Titian R. Peale, there are some patches of grow- 

 ing corals, and they are said to extend along to 20° or 21° S. 



The Bermudas are of coral origin, and are the most northern 

 point of growing reefs. 



In the West Indies, the reefs of Key West, Cuba, the Baha- 

 mas, and many of the eastern islands are well known. On the 

 east coast of Florida they continue up as far as Cape Florida, in 

 latitude 25° 40' N. : the west coast is free of them. There are 

 also said to be patches at intervals along the coast of Venezuela 

 and Guatemala ; but the west shores of the Gulf of Mexico, as 

 well as the northern, like West Florida, are mostly low, and eve- 

 rywhere without corals. They are within the influence of the 

 Mississippi and other large rivers. 



We have thus seen that the earth is belted by a coral zone, 

 corresponding nearly to the tropics in extent, and that the oceans 

 throughout it abound in zoophyte reefs, wherever congenial sites 

 are afforded for their growth. We have found that the currents 

 of extra-tropical seas, which flow westward, and are interrupted 

 and trended towards the equator by the continents, contract the 

 coral seas in width, narrowing them to a few degrees on the 

 western coasts of the continents ; while the tropical currents, 

 flowing eastward, diverge from the equator and cause the belt to 

 widen near the eastern shores. The polar currents flow also by 

 the eastern coasts, preventing the warmer waters from increasing 

 the width of the coral zone as much as it is contracted on the 

 western coasts. Moreover, the trend and capes of the coast pro- 

 duce other modifications in the direction of the currents, the 

 most of which are apparent in the actual distribution of coral 

 reefs. On the shores of the continents we have observed that 

 there are no extensive reefs, except along eastern Africa ; and, 

 while other lands abound in rivers, this African coast has only 

 some comparatively small streams. Thus the influence of con- 

 tinental waters and detritus on the distribution of reefs, has been 

 shown to be very marked. But about the Pacific islands, where 

 streams are small, the same cause has had little effect, seldom 

 doing more than modifying somewhat the shores and bottom of a 

 harbor. We have ascertained that in different groups, as the La- 

 drones, the Sandwich Islands, Samoa, New Hebrides, there is an 

 inverse relation between the extent of reefs and the evidences of 

 recent volcanic action in the island; and that the largest reefs 

 exist where there is no proof of former igneous action, or where it 

 has long ceased. The adverse influence of volcanic agency to the 



