670 GEOLOGY. 



organic compounds. The chlorophyl-forming plants are, therefore, 

 ]imited to such depths as are penetrated by the rays necessary for the 

 photosynthesis of organic matter. Vision is cut off within 200 to 300 

 feet, and most plant growth takes place above that depth. Photo- 

 graphic effects become feeble or inappreciable at 1000 to 1200 feet.^ 

 The photosynthesis of plants is chiefly aided by the lower and middle 

 part of the spectrum, while the ordinary photographic work is chiefly 

 done by the upper end, so that the photographic Hmit is below the photo- 

 synthetic limit. Microscopic plants are sometimes found lower than 

 these limits, but they may have been carried below their working limits 

 by currents or other incidental agencies. For all general purposes, 

 the limiting depth of living carbon-compounding plants may be set at 

 100 fathoms, as a generous figure — about the average depth of the 

 border of the continental shelf — while the vast majority flourish only 

 in the upper third of this depth. 



Life does not cease here, for the products of this surface- life sink 

 to greater depths and are fed upon by forms of sea animals that have 

 become adapted to the dark and cold abyss of the ocean. Obviously, 

 these deep-sea forms are a very distinct type of life, and constitute a 

 fauna of the most pronounced kind, the abysmal fauna. Another dis- 

 tinct fauna occupies the open-ocean surface, the pelagic fauna. Still 

 a third fauna occupies the shallow-water tract, whose bottom Ues within 

 the light zone — the photohathic zone — and embraces the animals that 

 are dependent on the plants of this zone, or on its light and warmth, 

 and that are more or less fixed to the bottom or confined to the zone 

 because their food is there. 



The physical plane of demarkation between the surface or pelagic 

 fauna and the abysmal fauna is much more distinct and more 

 fundamental than any that is found in ascending above the surface 

 of the sea. The habitat of the shallow-water fauna is Hmited below 

 by the darkness, limited above by the water-surface, limited at one 

 side by the land, and limited on the other side by the deep sea. It 

 is hemmed in vertically between two planes only a few hundred feet 

 apart. Laterally, it is confined to a narrow belt about the borders of 

 the continents and to the more or less land-girt epicontinental seas. 

 Its vertical limits are fixed, but its lateral extent varies with the rela- 

 tions of the sea to the surface of the continental platforms. 



' For data, see Walther's Eiujeitung in die Geologie, pp. 35-45. 



