FRESH-WATER ALG. 261 
stone rocks, ay, even whole territories of alluvial 
soil, have been in a great measure composed. 
In Virginia there are vast beds of silicious marl, 
composed of the skeletons of countless generations 
of diatoms ; and it is said that the towns of Rich- 
mond and Petersburg, in the same province, are 
built upon an enormous stratum of these plants, 
every cubic foot of which contains billions more 
than the living population of men that throng the 
streets above them. Extensive tracts covered 
with similar relics of a former age occur through- 
out Britain. The peat mosses of Ireland and the 
Highlands of Scotland abound with them, and 
hundreds of species have been found beautifully 
preserved in the vast amber beds of Prussia. The 
peculiar white powdery substance known by the 
name of Berg mehl, or mountain meal, found in 
Swedish Lapland, under beds of decayed moss, 
and mixed by the inhabitants with their food in 
times of scarcity, is composed of fossil diatomacee, 
several species of which are still living, and occa- 
sionally seen in this country. The /oss¢l flour 
which the Chinese mix with their wheat or rice on 
similar trying occasions ;1 the unctuous clay which 
1 The following particulars regarding Chinese fossil flour, adapted 
from Ehrenberg’s late great work, A/ikrogeologie, may beinteresting :— 
‘Various kinds of edible earth were known in China in very an- 
cient times, and it may be presumed that many of them are mixed 
or pure tripolitan fresh-water bioliths, z.2., species of earths or 
