INTRODUCTION 17 



not suffice as a basis for the nutrition of the enormous 

 mass of marine animal life which not only ranges 

 over the great surface but penetrates into the depths 

 of the ocean far beyond the reach of sunlight. The 

 balance is redressed by the inconceivably great bulk 

 of the pelagic flora, a department of the study of 

 Algae which has been so much neglected that there 

 is little beyond an outline knowledge of its extent. 

 Inhabiting the surface layers of the ocean from the 

 polar regions to the tropics there is an extensive 

 floating marine vegetation, consisting of individuals 

 each of microscopic dimensions, and only under 

 special circumstances conspicuous in the mass. In 

 the colder seas of the north and south the mass of 

 this flora is composed of Diatomaceoc (Figs. 60), 

 which occur in such numbers as to yield on tow- 

 netting a palpable scum, becoming felt-like in con- 

 sistency on drying. This living diatomaceous scum 

 inhabits the upper layers of the waters, and rains 

 down its dead in the form of siliceous shells on the 

 bottom, forming extensive deposits known as diato- 

 maceous ooze. Such deposits of marine and fresh 

 water diatoms not only occur now on the floor of the 

 ocean, but are preserved in rocks from the Cretaceous 

 period, and are found, in great extent, in deposits of 

 Tertiary and Quaternary age. While such Diatomacecc 

 occur in greatest abundance in these regions, they have 

 besides a wide range over the ocean surface, becoming 

 mixed in temperate seas with Peridiniece (Fig. 56), 

 which also are found in vast shoals. The Peridiniece 

 are a group of organisms that require for their eluci- 

 dation much more study than has been given to 



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