Ke) MICROSCOPIC PLANTS. [CHAP. 
animal kingdom, but their title to be regarded as 
plants has been long since thoroughly established. 
Fic. 12. Fic. 13. 
They are, of course, exceedingly minute; and it fre- 
quently happens that in summer ponds are dried up, 
and the Diatoms and other low plants are carried 
long distances by the wind. We may then look in 
vain among the dry mud of the late pond for active 
Diatoms. We shall find what appear to be their 
empty valves, the protoplasm having become dried 
up by the sun. Yet if these are again placed in 
water they will revive almost immediately, and as 
soon as the rainy season commences we may search 
in the refilled ponds and find Diatoms in their former 
abundance. 
Long after the plant itself is dead the flint valves 
retain their form, and consequently geologists find 
vast beds and strata of rocks composed entirely of 
them. The earth, called Tripoli, used in polishing, 
consists almost entirely of their empty cases. Dr. 
Hooker says: “The phonolite stones of the Rhine, 
and the Tripoli stones, contain species identical with 
what are now contributing to form a sedimentary 
deposit, and, perhaps, at some future period a bed 
of rock extending in one continuous stratum for 
400 measured miles. I allude to the shores of the 
Victoria Barrier, along whose coasts the soundings 
_ 
