The rocks which come next to the Laurentian in the order of time are those known as the Cambrian.  They are so called because they constitute a large portion of the mountains of North Wales, and it was there that their characteristics were first carefully studied by Professor Sedgwick.  In one of the strata of this formation—­the Harlech Grit—­what are known as “ripple-marks” are found, proving that parts of these rocks at the time of their deposition formed a sea-beach, and that consequently at this time, at the latest, the dry land had emerged from the ocean.  In these rocks there are also decided traces of Volcanic Action, which seem to indicate the existence of a Volcano similar to the recent “Graham’s Island.”  At this point a considerable advance in animal life is found.  The fossils comprise several corals, varieties of mollusca, and a class of crustaceans peculiar to the very early rocks—­the trilobites.

On the Cambrian rocks rest the formations known as Silurian, from the fact that they were first thoroughly examined in South Wales (Siluria) by Sir E. Murchison.  In these rocks many fresh varieties of invertebrate fossils are found, and the vertebrata make their first appearance, numerous remains of fishes having been discovered.  The earliest specimen was found in the Lower Ludlow beds at Leintwardine, while the Upper Ludlow formation contains an extensive bed composed almost entirely of fish-bones.  Immediately above this bed are found what seem to be traces of land-plants, in the shape of the spores of a cryptogamous plant.

The Silurian rocks are succeeded by rocks which present two distinct characters, but are probably contemporaneous, the Devonian and the old Red Sandstone.  The former seem to have been deposited in the bed of the sea, while the latter is a fresh-water formation.  In these decided remains of land plants are found, of which about 200 species have at present been discovered.  The old Red Sandstone is also peculiarly rich in fossil fish.  The first signs of coal appear in this series of rocks, but on a very small scale.

We now come to what are known as the Carboniferous rocks, of which the lower series is known as the mountain limestone, and above it come the “coal measures,” containing numerous beds of coal, sometimes of great thickness.  These beds have resulted entirely from the decomposition, under peculiar circumstances, of an enormous development of terrestrial vegetation.  They seem to have originated in vast swamps, subject to occasional flooding, and to alternate movements of upheaval and subsidence.  On these swamps there must have existed for ages a vegetation of whose luxuriance the richest tropical jungles of the present time can give us no idea.  They tell the tale of a time when the temperature of the earth, was uniformly high (since coal fields are found in high northern latitudes), when the atmosphere was charged with moisture, and probably contained a large proportion of carbonic acid.  In the coal measures we come upon the first traces of land animals.  Several remains of reptiles have been found, as well as footprints left on the soft mud or sand of a riverbank or sea-beach.  There seems to be no doubt that they were left by lung-breathing animals.