80 HISTORICAL PALAZZONTOLOGY. 
muddy sediments, the latter being sometimes red, but more 
commonly nearly black in colour. It has often been supposed 
that the Cambrians are a deep-sea deposit, and that we may 
thus account for the few fossils contained in them; but the 
paucity of fossils is to a large extent imaginary, and some of 
the Lower Cambrian beds of the Longmynd Hills would ap- 
pear to have been laid down in shallow water, as they exhibit 
rain-prints, sun-cracks, and ripple-marks—incontrovertible evi- 
dence of their having been a shore-deposit. The occurrence 
of innumerable worm-tracks and burrows in many Cambrian 
strata is also a proof of shallow-water conditions ; and the gen- 
eral absence of limestones, coupled with the coarse mechani- 
cal nature of many of the sediments of the Lower Cambrian, 
may be taken as pointing in the same direction. 
The Zife of the Cambrian, though not so rich as in the suc- 
ceeding Silurian period, nevertheless consists of representa- 
tives of most of the great classes of invertebrate animals. ‘The 
coarse sandy deposits of the formation, which abound more 
particularly towards its lower part, naturally are to a large 
extent barren of fossils; but the muddy sediments, when not 
too highly cleaved, and especially towards the summit of the 
group, are replete with organicremains. ‘This is also the case, in 
many localities at any rate, with the finer beds of the Potsdam 
Sandstone in America. Limestones are known to occur in 
only a few areas (chiefly in America), and this may account for 
the apparent total absence of corals. It is, however, interest- 
ing to note that, with this exception, almost all the other lead- 
ing groups of Invertebrates are known to have come into 
existence during the Cambrian period. 
Of the land- surfaces of the Cambrian period we know 
nothing ; and there is, therefore, nothing surprising in the fact 
that our acquaintance with the Cambrian vegetation is confined 
to some marine plants or sea-weeds, often of a very obscure and 
problematical nature. The “ Fucoidal Sandstone” of Sweden, 
and the “ Potsdam Sandstone” of North America, have both 
yielded numerous remains which have been regarded as mark- 
ings left by sea-weeds or “ Fucoids ;” but these are highly enig- 
matical in their characters, and would, in many instances, seem 
to be rather referable to the tracks and burrows of marine 
worms. ‘The first-mentioned of these formations has also 
yielded the curious, furrowed and striated stems which have 
been described as a kind of land-plant under the name of 
Lophyton (fig. 28). It cannot be said, however, that the vege- 
table origin of these singular bodies has been satisfactorily 
proved. Lastly, there are found in certain green and purple 
