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 
Algze 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 Diatomacew (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 Diatomacew 
occur in greatest abundance in these regions, they have 
besides a wide range over the ocean surface, becoming 
mixed in temperate seas with Peridinicw (Fig. 56), 
which also are found in vast shoals. The Peridinica 
are a group of organisms that require for their eluci- 
dation much more study than has been given to 
Cc 
