| Slates of Ireland.| | | e.  Conglomerates and| | | and Sandstones of | | | Sutherlandshire? | | \f.  Menevian Beds. | |

Like all the older Palaeozoic deposits, the Cambrian Rocks, though by no means necessarily what would be called actually “metamorphic,” have been highly cleaved, and otherwise altered from their original condition.  Owing partly to their indurated state, and partly to their great antiquity, they are usually found in the heart of mountainous districts, which have undergone great disturbance, and have been subjected to an enormous amount of denudation.  In some cases, as in the Longmynd Hills in Shropshire, they form low rounded elevations, largely covered by pasture, and with few or no elements of sublimity.  In other cases, however, they rise into bold and rugged mountains, girded by precipitous cliffs.  Industrially, the Cambrian Rocks are of interest, if only for the reason that the celebrated Welsh slates of Llanberis are derived from highly-cleaved beds of this age.  Taken as a whole, the Cambrian formation is essentially composed of arenaceous and 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 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.