Page:Popular Science Monthly Volume 51.djvu/574

560 region fully accessible to observation. The proofs of the existence of old volcanoes consist in part of the lavas, agglomerates, breccias, and tuffs erupted from the earth's interior. The walls of the orifices through which these materials have reached the surface are also to be found, but often more enduring than these is the column of hardened material filling the once belching chimney of the volcano. Molten material that does not reach the surface is often intruded from below into vertical fissures, forming dikes, or spreads out in horizontal cavities, forming sills and laccolites. Sir Archibald describes all these kinds of evidence in his early chapters, and then enters upon a description of the ancient volcanic localities of Britain in the order of the geological ages. The Lewisian gneiss in northwest Scotland affords glimpses of probable volcanic activity at the very beginning of geological history, and there is other similar evidence dating from pre-Cambrian times. In South Wales a remarkably varied display of British Cambrian volcanic rocks has been preserved. Coming to Silurian time, we find a singularly complete volcanic chronicle. In Merionethshire, Pembrokeshire, Caernarvonshire, Anglesey, the Lake District, and the eastern parts of Ireland the history of eruptions is unmistakable. During the Old Red Sandstone period groups of volcanoes rose in long lines from the waters of most of the lakes, and threw out lava and ashes over tracts hundreds of square miles in extent. Only the earlier half of the Carboniferous period was productive of volcanoes in Britain, but vast remains of these are to be seen in the puys and plateaus of Scotland and in numerous dikes, sills, and bosses. After the last of the Palaeozoic eruptions ages of quiescence elapsed before volcanic activity recommenced in Tertiary time. Remains of Tertiary volcanoes are to be found chiefly in the isles of Skye and Mull and in northeastern Ireland. In summarizing the studies here presented our author calls attention to the distribution of the ancient volcanic eruptions along the western margin of the European continent, thus conforming to the tendency of modern volcanoes to range themselves along continental borders. From Archæan time to the Tertiary volcanic activity was remarkably persistent in the British Isles. It is evident from a study of the British examples that where no fissures existed in the visible part of the earth's crust communication from the internal magma to the surface was effected in hundreds of instances by explosions which blew out an orifice. Besides the manner in which the vents were formed, the kind of materials ejected, the mode of closing of the chimneys, and the manifestations of the final efforts of the volcanic force are all to be read in the grand chronicle of the rocks. Seven colored maps and nearly four hundred figures illustrate the work.

While polar auroras may be classed among the more spectacular phenomena of the heavens, they do not force themselves upon our attention as storms, lightning, shooting stars, and eclipses do. They have been studied only under difficulties, for they do not occur frequently in temperate regions as lightning does, and the time when preparations should be made to observe them can not be foretold as with eclipses. It is interesting to have laid before us, in the latest volume of the International Scientific Series, the knowledge that has been gained in the face of such obstacles. Much