LIFE IN THE OCEAN CLARK 391 



The light gradually diminishes so that in the clearest and most 

 sunlit seas, at about 650 feet, there is only a pale moonlight at 

 noon on the brightest day, while at greater depths there is no light 

 at all. Wave motion dies away, and not far below the surface there 

 is perpetual quiet even in the fiercest hurricane. The temperature 

 declines, rapidly at first and then more slowly; in the abysses it is 

 mostly a few degrees above the freezing point — below it in some 

 places. The pressure increases so that at 15,000 feet it is about 2y 2 

 tons to the square inch. 



The number of kinds of animals found between tide marks, in 

 rock pools, on the beaches, or on piling, is relatively small, rapidly 

 increasing from the high to the low water mark. Below the low- 

 tide mark the variety of animal life is markedly increased. Beyond 

 a slight depth, 50 feet or so, but varying in different places, within 

 which there are often well-marked zones, some of the shore forms 

 disappear, but other creatures take their places and still others con- 

 stantly appear at greater depths. The maximum variety of marine 

 animal types is found on bottoms between about 600 and 1,200 feet, 

 where the light is dim to almost absent, the water is cool and very 

 still, and there is abundant food provided by the shore detritus and 

 the sea above. 



Within this zone there are many animals of large size, crabs 11 

 feet or more from claw to claw, huge urchins and starfish, great 

 plantlike things looking like small apple trees (Prvmnoa, etc.), 

 masses of large crinoids, stalked and unstalked, and other creatures, 

 and probably in certain places swimming about the giant squid and 

 cuttles. 



Below this zone the stillness of the water and the increasing pres- 

 sure favor the deposit of the finest silt, and the bottoms are chiefly 

 of fine mud, passing into the so-called oozes made up of the shells 

 of the millions and millions of small creatures constantly dying in 

 the layers above. The greater part of the sea bottom beyond the 

 coastal muds is formed of globigerina ooze, consisting of the shells 

 of minute shelled animals, the oceanic foraminifera, largely globi- 

 gerinas, with some bottom-living types and a few other things. 

 Less common are the pteropod oozes, made up of the shells of oceanic 

 mollusks, the radiolarian oozes, and the diatom oozes. Toward the 

 middle of the oceans the oozes gradually pass into an excessively 

 fine red mud, which is the typical bottom of all the abysses far from 

 land. 



On the red mud everywhere and sometimes on the oozes lie scat- 

 tered the ear bones of whales and the teeth of sharks, the only por- 

 tions of these animals that will persist indefinitely. Some of the 

 sharks' teeth on the red mud are of gigantic size, several inches in 

 length, and came from species long extinct but known as fossils else- 



