608 DYNAMICAL GEOLOGY. 



Secondly, by temperature and depth of water. The reef-forming 

 corals grow in the warmer ocean-waters, in which the mean tem- 

 perature for the coldest month does not fall below 68° F. The 

 limit in depth appears to depend on the degree of light and press- 

 ure for which the species were made. 



The following zones in depth have been recognized by Forbes and other ob- 

 servers for the convenience of marking the distribution of marine species : — 



1. The Littoral zone, — or the tract between high and low tide level. 



2. The Laminarian zone, — from low water to fifteen fathoms (90 feet). This 

 zone is so named from the fucoidal sea-weed, called sometimes Tangle-weed, 

 which is of the genus Laminaria, a plant especially of rocky shores. 



3. The Coralline zone, — from 15 to 50 fathoms. 



4. The Deep-sea Coral zone, — from 50 to 100 fathoms and beyond. 



The zones of oceanic temperature are marked on the Physio- 

 graphic Chart, and are explained on pages 42-44, where also facts 

 are mentioned illustrating the geological bearing of the subject. 



2. The nature of different organic products, and the fitness of the species 

 affording them for making fossils and rocks. 



(a.) Nature of the organic products contributed to rock-formations. — The 

 following are some of the general facts relating to the nature of the 

 organic products contributed by life to the rocks : — 



1. Plants afford coal, fossil leaves, and fossil wood. 



2. Animal remains are more or less durable according to the pro- 

 portion of stony ingredients present. 



3. Shells, corals, and the like contribute to rock-formations almost 

 solely carbonate of lime, or the material of limestones. 



4. Bones, in addition to carbonate of lime, contain much phos- 

 phate of lime and animal matter. 



5. Diatoms, Poly cystines, and spicula of Sponges afford silica. 



Facts relating to the change of wood to mineral coal are mentioned on page 359. 

 Mineral oil is another result of the decomposition of vegetation. When the 

 carbon is only sparingly diffused through earth, it gives it a blackish color, 

 which is lost when the material is highly heated. 



Plants also afford some sulphur, potash, and soda. Carbonic acid is one of 

 the important results of their decomposition. 



Some sea-weeds are calcareous like corals, owing to their secreting lime among 

 their sparse tissues. (See page 67.) 



Animal membranes decompose and pass off for the most part as gases. Some 

 of the carbon often remains in the bed in which it is buried, giving it a dark 

 color. Impressions of the soft parts of animals, as of Cephalopods, have been 

 found in rocks ; but they are very rare. The tissues that penetrate shells and 

 bones are sometimes in part retained by the ancient fossil. Two cases are 



