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 appear to have been laid down
in shallow water; as they exhibit rain-prints, sun-cracks, and
ripple-marks--incontrovertible evidence 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 general absence of limestones, coupled with
the coarse mechanical nature of many of the sediments of the
Lower Cambrian, maybe taken as pointing in the same direction.

The _life_ of the Cambrian, though not so rich as in the succeeding
Silurian period, nevertheless consists of representatives 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
organic remains. 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, interesting to note that, with this
exception, almost all the other leading groups of Invertebrates
are known to have come into existence during the Cambrian period.

Fig. 28.--Fragment of _Eophyton Linneanum_, a supposed land-plant.
Lower Cambrian, Sweden, of the natural size.

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 markings left by sea-weeds or
"Fucoids;" but these are highly enigmatical 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-plan