that the lowest forms of life should be the first and earliest to reveal itself to the lenses of the paleontologist !

Turning next to the CAMBRIAN system, so termed by Professor Sedgwick, because well developed in the region of Wales (the ancient Cambria), we find it for the most part made up of crystalline schists and slates, hard silicious grits, and altered limestones. Like the Laurentian, its strata have undergone extensive metamorphism or internal mineral change, and like the Laurentian, too, it is frequently intersected by veins and eruptive masses of granite, and felspathic greenstone. On the whole, however, its sedimentary character is much more apparent; its micaceous schists are interstratified with grits and sandstones, and its slates often earthy and more distinctly laminated. Taking it in the mass of its 15,000 or 20,000 feet, it has altogether a more recent aspect than the Laurentian, and bears, not only in its sandstones and shaly slates, but in its imbedded pebbles, ripple-marks, and tracks of marine worms, more decided evidence of its aqueous origin. Of course, like other formations, the Cambrian will vary in composition in different regions, sometimes being more slaty, and at others more schistose and crystalline; but slaty beds, micaceous flagstones, gritty sandstones, and limestones more or less crystalline, may be taken as the normal aggregate.

When we turn to the fossils of the system we find them also more numerous and intelligible than those of the Laurentian, partly, no doubt, from the less metamorphism the beds have undergone, but chiefly, perhaps, from that advancing development of life we are accustomed to associate with the newer and newer formations. At all events, instead of a single organism, as in the Laurentian system, we now find, if not a numerous, at least a fair array of zoophytes, echinoderms, shell-fish, annelids, and crustacea. The species are

certainly not the highest of their respective orders, neither are they exactly the lowest ; but, making ample allowance for the defects of our present information, it may be safely asserted that the fauna or animal-life of the Cambrian period is altogether one of a lowly character, and that of the flora or plant-life we know nothing beyond a few indistinct impressions of algae or sea-weeds. Even the little we now know of the Cambrian flora and fauna was altogether unknown five-and-twenty years ago; and it is chiefly since 1846, and more especially since 1859, that its fossiliferous character has been fairly established.

Here then, as in the case of the Laurentian system, we have a long period of the earth's history-so long that 18,000 or 20,000 feet of sediment was accumulated in certain parts of the ocean-and of which we know nothing beyond what is recorded by these marine strata and the fossils they contain. We have no glimpse of the land from which these sediments were worn and wasted; yet there must have been broad lands to supply this waste, and rivers to transport it. We are utterly ignorant of the plant-life and animal-life—if terrestrial life then existed-by which these lands were peopled. We know nothing of the disposition of sea and land as compared with the existing continents and seas. All that we clearly perceive is the existence of these sediment-receiving oceans, with their scattered sea-weeds, zoophytes, shell-fish, and crustacea, and only faintly and at distant intervals their sandy and muddy shores, in which annelids bored and left their burrows, and over which crustacea tracked and left their traces. Little as this knowledge may seem, it is everything compared with the beliefs of our ancestors; it is a great deal compared with what was known even by the last generation; and it holds out the encouragement that another generation, by following in the right path, will arrive at a fuller in

sight into the physical and vital aspects of this primary period.

The last of the primary systems which form the subjects of our Sketch is the SILURIAN, so named by Sir Roderick Murchison, because well displayed and first examined by him in that border country between England and Wales which in ancient times was inhabited by the Silures. Rocks of Silurian age have been found in almost every region of the globe-in Central and Northern Europe, largely in both Americas, and in Australia-and though they differ much in their mineral composition, some being more crystalline and slaty than others, still on the whole there is a wonderful similarity among them, both in their lithological and fossil aspects. Occasionally they are so metamorphosed as to be undistinguishable from the crystalline schists of the Cambrian and Laurentian; but, generally speaking, their sedimentary character is abundantly apparent in the numerous alternations of sandstones, slaty shales, and limestones, and we see in these strata, with their fossil corals, shells, and crustacea, the clearest evidence of deep and widespreading seas. Altogether the geological record becomes more legible, and we can form some notion of the earth's terraqueous conditions during the long and gradual deposition of the Silurian sediments. We say long and gradual deposition, for in our own islands these strata are from 20,000 to 30,000 feet in thickness, embracing numerous alternations of rock-material, and repeated removals and renewals of genera and species of animals.

In these Silurian strata we perceive limestones formed of coral-reefs and calcareous debris, slates, and slaty shales arising from the deeper sea-muds, and sandstones, grits, and conglomerates composed of the sands and pebbly shingle of the shallower waters. Here and there through the mass we

find interstratified overflows of lava and showers of volcanic ashes, indicating that then, as now, the vulcanic forces were active in their work of upheaval and eruption. How wonderfully well these old rocks have retained the record of their history! Here shore-formed sandstones pattered by crustacean feet and riddled with worm-burrows; there limestones formed of coral-growth and shell-debris in the deeper waters here slaty shales composed of the slimy mud of the stiller depths, and replete with zoophytes; and there at intervals vast sheets of tuff and porphyry that had been showered abroad as ashes or ejected as lava from submarine volcanoes. Occasionally, in the pebbles of the conglomerates, we catch glimpses of the kind of rocks that formed the lands from which these sediments were transported; and from drifted clubmoss-like twigs in the shales, we know that these lands were clothed in some degree with a vegetation, however lowly.

Such is the tale told of the Silurian epoch by its own rockformations; but the history receives a deeper and livelier interest when we come to consider the number and variety of organisms imbedded throughout the system. It is true that these are of lowly orders and wholly invertebrate, if we except a few scattered fish-remains found in the very uppermost beds, and by many regarded as belonging more properly to the Old Red Sandstone; but lowly as they are, they occur in vast exuberance and variety, and mark a marvellous progress in life-development compared with what is known of the Cambrian and Laurentian. Sea-weeds and drifted twigs and spores of clubmoss-like land-plants is all we know of the Silurian flora; but certain beds of anthracite and anthracitous shales favour the idea that plant-life was then more exuberant than has yet been detected. In its fauna or animal-life we have foraminiferal organisms, sponges, corals, polyzoa, or aggregate animals like the sea

mats and sea-pens (graptolites, &c.); shell-fish of every order, bivalve and univalve, deep-sea and shore dwellers; radiate animals, like the encrinites and the star-fishes; sea-worms in their tracks and burrows; and crustaceans, chiefly trilobites and eurypterites, having some resemblance and affinity to the existing limulus or king-crab. These organisms are not found indiscriminately throughout the system, but vary in number and distribution according to the kind of stratum in which they are imbedded—every zoophyte and shell-fish preferring a certain kind of sea-bottom; and according as they occur in the lower, middle, or upper portion of the system-the upper being the more prolific and characterised by the higher species. Numerous and varied as they are, they are (with the exception of the obscure land-twigs) exclusively marine; and if we regard the uppermost beds, with their fish-remains, as the base of the Old Red Sandstone, they are entirely invertebrate,* and mark, so far as our present evidence goes, the close of a long primary cycle, during which vitality was gradually evolving, in a fixed and definite order, from lower to higher manifestations.

As economic repositories these primary systems are, in some regions, of considerable importance; less, however, for their rock-products than for the metalliferous veins by

*The reader must guard against the idea that there are any sharp lines of demarcation between the so-called Systems of geologists. The life of certain estuaries and seas may no doubt be brought to a close by some sudden catastrophe, but such breaks are merely local, and do not affect the general life-arrangements of the globe. When we speak, therefore, of the Silurian as "marking the close of a long invertebrate period," it is not meant to be asserted that there were absolutely no fishes during the deposition of the uppermost Silurians, but simply that the Primary Periods as a whole were characterised by their want of vertebrate remains; and that the strata in which fish-remains do occur may be regarded, without detriment to the science, either as uppermost Silurian or as lowermost Old Red Sandstone.