INTRODUCTION TO CRYPTOGAMIC BOTANY. 127 



liand, they do not altogether avoid thermal springs, they 

 abound amongst the pancake ice of the South Pole, as far 

 south as seventy-eight degrees, where they must occasionally be 

 subject to very low temperature. " Though much too small to 

 be discernible to the naked eye, they occur in such countless 

 myriads, as to stain the berg and pack ice, wherever they aie 

 washed by the swell of the sea; and when inclosed in the con- 

 gealing surface of the water, they impart to the brash and 

 pancake ice, a pale ochreous colour/' As the siliceous coats 

 are indestructible under ordinary circumstances, even when 

 these productions have been swallowed by animals, they 

 are readily distinguished, either on examination of the contents 

 of the stomach or of voided excrement. Consequently, where- 

 ever these substances are deposited, they are to be found, as in 

 the penguin rookeries, and other sources of guano, where they 

 remain for ages, having been in the first instance swallowed with 

 the mollusca, of which the food of the birds mainly consists. The 

 shells of the dead animals also gradually subside, and in conse- 

 quence, myriads are in some situations brought up with sound- 

 ings. It can be no matter of surprise, therefore, that these 

 bodies, though so minute, are perfectly preserved in a fossil state 

 in several strata, in some instances in such abundance, that they 

 are collected and sold under the name of Tripoli polishing 

 powder, for which purpose they are admirably adapted. Dr. 

 Ehrenberg even asserts, that species are to be found in a living 

 state in situations where they have been propagated from times 

 far anterior to the present modification of the earth, to which 

 man has been assigned as an occupant ; but such fancies require 

 the very strongest proof before they can be received as certain 

 verities. Some notion of the extent to which they are depo- 

 sited in modem days may be conceived from the facts adduced 

 by Dr. Hooker. " The Phonolite stones of the Rhine and the 

 Tripoli stone, 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 four hundred measured miles. I allude to the 

 shores of the Victoria barrier, along whose coasts the soundings 

 examined were invariably charged with Diatomaceous remains, 



