of curved spines and shagreen-like fragments. The creatures of which these are relics appeared as forerunners of the remarkable assemblage of fishes in the next geological period (see p. 260). All the animal remains hitherto enumerated are relics of the inhabitants of the sea. Of the land-animals of the time nothing was known until the year 1884, when, by a curious coincidence, the discovery was made of the remains of scorpions in the Silurian rocks of Sweden, Scotland, and the United States, and of an insect allied to the living cockroach (Palæoblattina) in those of France. If scorpions and insects existed during this ancient period we may be sure that other forms of terrestrial life were also present. A new interest is thus given to the prosecution of the search for fossils among the older formations.

Putting together the evidence furnished by the rocks and fossils of the Silurian system, we get a glimpse of the aspect of the globe during the early geological period which they represent. The rocks bring before us the sand, mud, and gravel of the bottom of the sea, and tell of some old land from which these materials were worn away. The detritus carried out from the shores of that land was laid down upon the sea-bottom just as similar materials are being disposed of at the present day. The area occupied by Silurian rocks marks out the tracts then covered by the sea. Following these upon a map we perceive that vast regions of the existing continents were then parts of the oceanfloor. In Europe, for example, Silurian rocks underlie the greater part of the British Islands, whence they stretch northwards across a large part of Scandinavia and the basin of the Baltic. They rise to the surface in many places on the continent from Spain to the Ural Mountains. They are found forming parts of some

of the great mountain-chains of the globe, as, for instance, in the Cordilleras of South America, in the Alps, and in the Himalayas. Even at the antipodes they are met with as thick masses in Australia and New Zealand. It is evident that the geography of the globe in Silurian times was utterly unlike what it is now. A large part of the present land was then covered with shallow seas, in which the Silurian sedimentary rocks were laid down. There would seem to have been extensive masses of land in the boreal part of the northern hemisphere connecting the European, Asiatic, and American continents. Along the coast-line of the

FIG. 130.-Silurian Cephalopods. (a) Orthoceras emeritum (3); (b) Trochoceras (Lituites) cornu-arietis (3).

northern land and across the shallow seas lying to the south of it, the same species of marine organisms migrated freely between the Old and the New Worlds.

The following Table shows the subdivisions which have been made in the Silurian system of Britain.

Ludlow group (mudstone and Aymestry limestone)-Kirkby Moor and Bannisdale flags and slates.

Wenlock group (shales and limestones)-Denbighshire and Coniston grits and flags. Upper Llandovery group-May Hill sandstones.

Lower Llandovery group-grits and sandstones.

Bala and Caradoc group-sandstones, slates, and grits, with Bala (Coniston) limestone.

Llandeilo group-dark argillaceous and sometimes calcareous flagstones and shales.

Arenig group-dark slates, flags, and sandstones.

S

CHAPTER XIX

DEVONIAN AND OLD RED SANDSTONE

THE DEVONIAN system, which comes next in order, was named by Sedgwick and Murchison after the county of Devon where they studied its details. In Europe, and likewise in the eastern part of North America, it occurs in two distinct types, which bring before us the records of two very different conditions in the geography of these regions during the time when the rocks composing the system were being deposited. The ordinary type, which occurs all over the world, represents the tracts that were covered by the sea, and has preserved the remains of many forms of the marine life of the period. It is that to which the name Devonian is more particularly applicable. The less frequent type is characterised by thick accumulations of sandstones, flagstones, and conglomerates that were laid down in lakes and inland seas, and contain a distinct assemblage of land and fresh-water fossils. This lacustrine type is known by the name of OLd Red SandstONE.

In their general character the Devonian rocks resemble those of the Silurian system underneath. In Central Europe, where they attain a thickness of many thousand feet, their lower division consists mainly of sandstones, grits, greywackes, slates, and phyllites. The central zone contains thick masses of limestone, often full of corals and shells, while the upper portions comprise thin-bedded sandstones, shales, and limestones. These various strata represent the sediments intermittently laid down upon the bottom of the sea which then covered the greater part of Europe. Here and there, they include bands of diabase and tuff, which show that submarine volcanic eruptions took place during their deposition.

In the north-west of Europe, however, the floor of the Silurian sea was irregularly ridged up into land, and large lakes were

formed, into which rivers from the ancient northern continent poured enormous quantities of gravel, sand, and silt. The sites of these lakes can be traced in Scotland, the north of England, and Ireland. Similar evidence of land and lake-waters is found in New Brunswick and Nova Scotia. That some of the larger lakes were marked by lines of active volcanoes is well shown in Central Scotland, where the piles of lava and ashes left by the eruptions are more than 6000 feet thick.

The occurrence of both marine and lacustrine deposits is of the highest interest, for, on the one hand, we learn what kinds of

FIG. 131.-Plants of the Devonian period. (a) Psilophyton (3); (b) Palæopteris (1). animals lived in the sea in succession to those that peopled the Silurian waters, and, on the other hand, we meet with the first abundant remains of the vegetation that covered the land, and of the fishes that inhabited the fresh waters. The terrestrial flora of the Devonian period has been only sparingly preserved in the marine strata; but occasional drifted specimens occur to show that land was not very distant from the tracts on which these strata were laid down. In the lacustrine series or Old Red Sandstone of Britain more abundant remains have been met with, but the chief sources of information regarding this flora are to be sought in New Brunswick and Gaspé, where upwards of 100 species of plants have been discovered. Both in Europe and in North America, the Devonian vegetation was characterised by the predominance of ferns, lycopods (Lepidodendron, etc.), and calamites. It was essentially acrogenous-that is, it consisted

mainly of flowerless plants like our modern ferns, club-mosses, and horse-tail reeds. Traces of coniferous plants, however, show that on the upland of the time pine-trees grew, the stems of which were now and then swept down by floods into the lakes or the sea.

While the general aspect of the flora was uniformly green and somewhat monotonous, the fauna had now become increasingly varied. We know that these early woodlands were not without insect life, for neuropterous and orthopterous wings have been preserved in the strata. Some of these remains indicate the existence of ancient forms of ephemera or May-fly, one of which was so large as to have a spread of wing measuring 5 inches across. There were likewise millipedes, which fed on the decayed wood of the forests. Traces of land-snails too have been detected among the fossil vegetation. It is evident, however,

that the plant and animal life of the land has only been sparingly preserved; and though our knowledge of it has in recent years been largely increased, we shall probably never discover more than a mere fragmentary representation of what the original terrestrial flora and fauna really were.

The lake-basins of the Old Red Sandstone have yielded large numbers of remains of the fishes of the time. These are members of the remarkable order of Ganoids - the earliest known type of fishes-which, though so abundant in early geological time, is represented at the present day by only a few widely scattered species, such as the sturgeon, the polypterus of the Nile, and the bony pike or garpike of the American lakes. These modern forms are denizens of fresh water, and there is reason to believe that their early ancestors were also in

habitants of lakes and rivers, though many of them may also have been able to pass out to the sea. The ganoids are so named from the enamelled scales and plates of bone in which they are encased. In some of the fossil forms, this defensive armour consisted of accurately fitting and overlapping scales (Figs. 132, 133); in others, the head with more or less of the body was protected by large and thick plates of