56 PHYSIOGRAPHIC GEOLOGY. 



there is of light at abyssal depths ; but this is not probable. Murray observes that the 

 eyes of the Fishes are in general unusually large at depths of 480 to 1200 feet; but 

 beyond this depth, small-eyed species as well as large-eyed are common, and many also 

 are blind. 



llie Littoral zone. — By means of light, rather than temperature, the limits of the 

 Littoral zone are determined. Reef-forming Corals, and therefore coral-made reefs, have 

 their limit in depth at 120 or 150 feet in the equatorial regions, and at 90 to 96 feet near 

 the border of the coral-reef seas ; and since the temperature of 68° F. in the tropics is 

 usually 600 feet below the surface, light is made, by Fuchs, to be the chief cause. A 

 vast variety of species are congregated under like limitation, the coral-reef seas being 

 "the gathering grounds of an extremely rich fauna," says Fuchs, and one so peculiar that 

 the terms coral-fishes, coral-moUusks, and the like, would not be inappropriate ; a fauna 

 that embraces " the whole splendor of the animal life " of the Indian and Pacific oceans. 

 Within this Littoral zone belong, moreover, the areas of large Bivalves, such as Oysters, 

 Pearl Oysters, Scallops, which have their maximum development in from 48 to 60 feet, 

 and are not found below 120 feet.i 



The seaweed areas reach to a depth of 30 fathoms, or about 200 feet. Plants are 

 dependent on light for assimilation, and hence comes this narrow limit for the aquatic part 

 of the vegetable kingdom. 



The Fucoids and strap-like Laminarians, or the brown and olive Seaweeds — related, 

 it is supposed, to the Seaweeds of early time, when no seas were colder than those of the 

 modern temperate zone — live now on most shores from the tropics to the poles, and attain 

 their greatest size in the colder latitudes. 



The only deep-water plants thus far observed are Corallines at 900 feet, and small 

 Algse, found to bore into corals that came up, according to Duncan, from a depth of 6000 

 feet. 



The depths. — Below 420 to 480 feet are the regions of darkness. They are divided 

 locally into two sections, a warm and a frigid, by the Gulf Stream and other tropical 

 currents. The depth to which the warm waters of this stream extend along the borders 

 of the Atlantic basin, and thence across the ocean to Great Britain, are mentioned on 

 page 5. Where these waters wash the sides of the Atlantic basin from Florida north- 

 eastward, there is a profusion of life of all marine kinds ; and the same is true for the 

 area within the British sea. 



The cold belt passes close by the western side of the warm belt off New Jersey and 

 Nantucket. The commingling of the two in a storm is stated by Professor Verrill (1882) 

 to have probably caused the extermination — only temporary, it has proved — of a large 

 food- fish, the Tile-fish, of the genus Lopholatilus, which was caught abundantly by a Fish 

 Commission expedition in 1881 off Nantucket with a trawl at a depth of 420 to 900 feet. 

 During the following winter great numbers of the dead Tile-fish were seen by passing 

 vessels, floating at the surface ; and in the dredging of 1882 over the same area not one 

 was obtained ; and many other species dredged in 1881 were missing in 1882. In 1890 

 the Tile-fish was again found. 



The bottom of the ocean through all its depths is constantly receiving contributions of 

 the hard parts of its living species, from the bones of Whales and Sharks to the siliceous 

 shells of Diatoms, and the calcareous of Rhizopods. It is often difficult to determine for 

 the smaller species whether they are denizens of the dark depths or of more superficial 

 waters. Most of the pelagic species that are found abundantly at the surface of the 

 ocean during the dark hours of the night, and may then be easily taken with a hand-net,, 

 go to greater depths during the day, showing that they are really part of the fauna of the 

 darkness. Pelagic species, according to Agassiz, are mostly confined to within 1200 feet 

 of the surface. 



1 J. Fuchs, Geol. VerhandL Eeichanstalt, No. 4, 1882; Ann. Mag. If. Hist., January, 1883. 



