154 GEOLOGY. 



rial of the rocks. Especially is this the case with the 

 shells. 



237. Coral Reefs. Along many shores there are ridges 

 or reefs made by coral animals. The mode of their con- 

 struction I will describe. These animals do not begin 

 their work on the edge of the land, for they like clear 

 water, which can not be had close to the shore. But 

 there is a limit away from the land beyond which they 

 will not work, and this limit depends on the depth of 

 water. They can not live beyond a depth of about 100 

 feet, and most often they choose a depth of 20 or 30 feet 

 to begin their operations. Where the water deepens rap- 

 idly from the shore, they are not as far away from it as 

 where it deepens gradually. Coral animals are confined 

 to certain climates. Almost all the shelving shores in 

 tropical seas are lined by the reefs which they make. 

 There is a remarkable exception in the case of the west- 

 ern coast of South America. Here there are no corals, 

 while they abound on the eastern coast in the same lati- 

 tudes. The reason is that the western coast has sweep- 

 ing along it a cold current from the antarctic regions. 

 Many of the coral reefs are not yet raised up to the sur- 

 face, and these hidden reefs are very dangerous to nav- 

 igation. Some reefs are of very great extent. Along 

 the coast of New Holland there is one over 1000 miles 

 in length, and for 350 miles in one part of it there is no 

 passage through it. Reefs are spoken of as being of two 

 kinds, fringing and barrier ; the former being near the 

 shore, and the latter at some distance from it, with a deep 

 channel between. The fringing reefs are very apt, after 

 a time, to become a part of the main land. The reason 

 is that there is a continual washing of material into, the 

 space between the land and the reef, both from the land 

 and the sea, chiefly the former. Indeed, Florida was all 

 made in this way, reefs having continually formed age 

 after age, and then joined to the main land. Many isl- 

 ands have been formed by coral animals. A large part 



