1002 PRINCIPLES OF STRATIGRAPHY 



often hold on to objects small enough to be carried away with 

 the seaweed during a violent storm ; and since the stipe and frond 

 of the seaweed are commonly covered with sedentary benthonic ani- 

 mals, this seaweed forms a ready agent for the wider dispersal of 

 such organisms. 



In general the larger algx are attached to a rocky or other 

 hard substratum or to other algae (epiphytic). On muddy or 

 sandy bottoms algae are rare, though stranded alg?e may be buried 

 in numbers in mudflats. The large algas (macrophytes) are mostly 

 restricted to the photic region, where they are distributed in the 

 two belts, the perpetually submerged (i. e., below low tide) and the 

 periodically emerging belt (between tides). Marine algae are found 

 to the height of the salt spray on the shore, while terrestrial algse 

 are known from the tropics, where they live as epiphytes on leaves, 

 especially in the rainy districts. In the temperate regions they are 

 associated symbiotically with fungi to form lichens, which increase 

 in numbers and importance as the climate becomes cooler. They and 

 the mosses constitute the chief epiphytes and epiliths in the tem- 

 perate and cooler climates. Lichens sometimes form structures 

 which under favorable conditions may be preserved. In the arctic 

 regions microscopic red and brown algre (Spha?rella, etc.) often 

 color the snow and ice and, together with many other microphytes, 

 form a characteristic element of the vegetation of these regions. 



Diatoms are important rock-builders, since their siliceous skele- 

 tons or frustules are readily preserved. They occur both in fresh 

 and salt water, no ditch, pond, or pool being without them, and they 

 form a characteristic member of the marine plankton. A great 

 many types, however, are benthonic, forming yellowish-brown 

 films on the mud in shallow pools, or growing attached by slender 

 stalks to other plants. They commonly possess the power of mo- 

 tion found also in desmids and other unicellular plants. Fossil 

 diatoms are abundant in the Tertiary of many localities, often form- 

 ing extensive beds of nearly pure frustules, generally of fresh 

 w^ater origin. L^nderlying the city of Richmond, Virginia, is a 

 bed of these organisms eighteen feet thick, while other extensive 

 deposits occur in the coastal plain of Maryland and New Jersey, 

 as well as in many other parts of the world. In Mesozoic deposits 

 they are less abundant, and they are not known positively from 

 Palaeozoic deposits, probably owing to alteration of the frustule. 

 Diatomaceous deposits are often erroneously spoken of as infusorial 

 earth. 



Fungi. The members of this group are destitute of chlorophyll 

 and, consequently, are dependent upon organic matter for food, be- 



