a tribe of ancient Britons), the people formerly inhabiting the district; sometimes they denote typical features of the formation: but they are the accepted nomenclature in treatises on geology, and are therefore adopted here.

I. PRIMARY.-The Laurentian rocks, vast and venerable sediments of primeval seas, are highly metamorphic. Heat, moisture, and enormous pressure have changed their sandstones into sparkling crystalline rocks, and their limestones into veined and variegated serpentines.

Formerly they were classed as ' Azoic '-i.e. without life; but of late years those which form the Laurentian Mountains in Canada, whence the general name of the series is derived, have acquired special interest from the discovery of certain veined structures in them, pronounced by some authorities' to be the remains of a large foraminiferal animal which has been named Eozoon Canadense. The foraminifera form perforated shell

1 Notably Professor Sir J. W. Dawson and the late Dr. W. B. Carpenter. The evidence against its organic character is given in Professors King and Rowney's Old Chapter of the Geolog. Record (Van Voorst, 1881). Cf. also Heilprin's Distrib. of Animals, pp. 134, 196, 236.

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coverings of exquisite symmetry and beauty from the lime which they secrete from the water.

1

'The longevity of an organic type has, on the whole, been in inverse proportion to its perfection;' and some of the lower types may smile at man's 'claims of long descent,' for they have survived through the long and change-bringing millions of years to this day, shedding their shells on the ocean floor, as their ancestors shed

FIG. 6. Foraminifer, Globigerina bulloides, magnified seventy diameters. This form is found floating in tropical and temperate seas.

theirs, forming vast chalk and limestone hills and. mountain ranges in relatively shallow seas. Not in deep oceans, as was formerly held, since the fossils are shown to resemble present shoal water deposits rather than similar oozes found in water over a thousand fathoms deep, which further confirms the theory that the great ocean beds have never been upraised. While some secrete chalk, others secrete flint. Among the latter are the minute plants known as diatoms, whose remains

compose, among other deposits, the 'rotten-stone' used as polishing powder, of which no less than forty-one thousand million skeletons go to make up a cubic inch.

It matters little if the organic character of Eozoon Canadense be finally disproved, for plentiful traces remain that the Laurentian waters swarmed with living things of low type but enormous size. The limestones, the

abundant graphites, and the great beds of iron ore which occur in its rocks are due to the action of animal and plant life.

as

The Cambrian rocks, although less metamorphic, add little to our knowledge of primitive plant-forms, such are preserved being probably algæ, or seaweed, corresponding to the tangles covering large areas of the Atlantic, especially the region called the Sargasso Sea. But the system is fairly rich in fossils of marine animals, themselves the descendants of a long line of perished ancestors. Sponges, sea-lilies, and low forms of mollusca, or true shell-fish, are found; but the typical and most perfect fossil is that of the three-lobed crustaceans called trilobites, which swarmed in those ancient seas, and survived till the Carboniferous period.

The Silurian rocks, although exhibiting in crumpled and rugged mountain chains the action of agents both above and below the earth, are much less metamorphosed than the preceding systems. They are in large measure the worn fragments of land areas which stretched across Northern Europe for above two hundred miles into the Atlantic, the sediment being deposited in a shallow sea which then covered Central and Southern Europe, and the floor of which was slowly raised as a primitive European continent at the close of the Silurian period by subterranean movements. The land plants, which are the earliest as yet met with, are allied to huge clubmosses, ancestors of the gigantic forest-kings of Devonian and Carboniferous times. The most ancient of all known land animals is a scorpion found in the upper Silurian beds of both Scotland and Sweden; while the marine remains are varied and numerous, comprising seaweeds, foraminifera, corals, star-fish, shell-fish of every kind, 1 Smithsonian Report, 1884, p. 604.

trilobites, and huger lobster-like crustaceans, sometimes measuring above six feet in length.

But the most important fossils are those of the earliest known vertebrates, in the form of armoured fishes,' allied to the sturgeon, and called ganoids (Gr. ganos, splendour; and eidos, form), from the brilliancy of their enamelled scales.

In this seemingly sudden appearance of highly organised animals marking so great an advance in struc

FIG. 11.-A, Recent Ganoid Fish; B, Ganoid from Devonian strata. ture on the higher invertebrates, the imperfection of the geological record is brought home to us. For if later forms are modified descendants of earlier, then not only are the transitional ancestral forms of the ganoids missing, but the species itself is enormously older than the fossils imply. The inquirer, however, need not despair, for only a limited portion of the dry land has as yet been explored with any completeness, and there are vast fossilholding areas submerged and inaccessible; yet one by

1 Discovered by Professor Claypole near the base of the Upper Silurian in Pennsylvania; cf. Smithsonian Report, 1884, p. 622.