612 DYNAMICAL GEOLOGY. 



3. By light. — Light is the chief agent determining distribution in 

 depth (Fuchs), temperature being subordinate to it. 



Experiment has proved the presence of light to a depth of 43 to 50 fathoms, and the 

 probability that only a little passes beyond this limit. To a depth of 40 fathoms, the 

 species, as announced by Fuchs, are species of the liyht, while those below are relatively 

 species of the darkness. The two ranges of species differ, although there is some com- 

 mingling, especially between 30 and 90 fathoms. But each of these ranges of species is 

 divided into a warmer-water and a colder-water section, as above explained. 



Many of the deep-sea species are blind. But crabs with good eyes live at a depth of 

 10,000 feet, and shrimps at 17,700 feet. Since large numbers of the species (Star-fishes, 

 Alcyonoid Corals, Crustaceans, Worms) are phosphorescent, the deep depths have been 

 supposed to be lighted by this means. 



Plants are eminently species of the light; sea-weeds grow mostly within 10 fathoms 

 of the surface, and rarely beyond 30. A few cases have been reported of the simplest 

 kinds of plants occurring at great depths. 



Some continental species living in dark places, as caverns, are blind ; 

 e. g., the blind Fish and Crustaceans of the Mammoth Cave, etc. 



4. By freedom from rough mechanical agents, and the reverse. — The 

 occurrence of the siliceous sponges especially over the bed of the deep 

 oceans, has been accounted for on the view that they are too delicate 

 to exist where there is much movement in the waters. On the other 

 hand, some Corals and other species seem to thrive best amid the 

 breakers. 



5. By the character of the bottom or shores, whether rocky, sandy, or 

 muddy. 



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. — 

 Some of the general facts, relating to the nature of the organic prod- 

 ucts contributed by Life to the rocks, are mentioned on pages 59 to 

 62. The following are additional facts : — 



Plants afford, besides carbon, oxygen, hydrogen, potash, and soda, with some sul- 

 phur and nitrogen. Carbonic acid is one of the important results of their decomposi- 

 tion. 



Animal membranes and oil decompose, and pass off for the most part as gases. Por- 

 tions of the carbon and hydrogen often remain in the bed in which they are bivried, 

 giving it a dark color, or making sometimes mineral oil or coal. Impressions of the 

 soft parts of animals, as of some Cephalopods, and the membranous part of the wings 

 of Pterodactyls, have been found in rocks; but they are very rare. 



The tissues that penetrate shells and bones are sometimes in part retained by the an- 

 cient fossil. Two cases are mentioned by Barrande, of the conversion of the animal 

 material, within a Lower Silurian Orthoceras, into adipocere (an animal substance 

 having the appearance of spermaceti); and he speaks of them as the oldest mummies 

 ever exhumed. 



A small percentage of phosphates and fluorids is derived from decomposing animal 

 tissues. 



