The next great geological period, to which Murchison gave the name of Silurian, has in Britain a fuller record than the period which preceded it. The rocks that tell its history are more varied in origin and structure. They are displayed at the surface over a far wider area, and, what gives them special interest and value, they contain a much larger assemblage of organic remains. For the immediate subject of the present volume, they have likewise the additional attraction that they include a singularly complete and widespread volcanic chronicle. They display in many admirable sections the piled-up lavas and tuffs of scores of volcanoes, scattered all over the three kingdoms, from the headlands of Kerry to the hills of Lammermuir. They thus enable us to form a truer conception of what the early Palæozoic volcanoes were than is possible from the more limited evidence furnished by the Cambrian system. At the beginning of the Silurian period most of the area of the British Isles lay under the sea. But if we may judge from the sedimentary strata which represent the floor of that sea, the water, during most of the time, was of no great depth. There is evidence, indeed, that during a part of the period the sea was deep enough to admit of the accumulation of wide tracts of radiolarian ooze, with but little admixture of mechanical sediment. But, for the most part, sand and mud were drifted from neighbouring lands, the more important of which probably lay to the north, over what are now the Highlands of Scotland and the north and north-west districts of Ireland. No general change in topography or in physical conditions took place at the close of Cambrian time. The older era glided insensibly into the newer, unmarked by any such catastrophe as was once supposed to have intervened at the end of each great geological period. There are traces, indeed, of slight local disturbances, but these only make the general gradual transition more marked. Of the vegetation which covered the Silurian lands hardly anything is known. Traces of lycopods and ferns have been detected, and these probably formed the chief constituents in what must have been rather a sombre and monotonous flora. The character of the terrestrial fauna is still hidden from us, though we do know that insects winged their way through those green flowerless forests, and that scorpions likewise harboured there. That these primeval arachnoids were air-breathers is shown by their breathing stigmata; and from the fact that they possessed a well-developed poison-gland and sting, we may believe that there were already living at the same time other land-animals, possibly of higher grade, on which they preyed. But of these ancestral types no actual relics have yet been discovered.