Popular Science Monthly/Volume 51/August 1897/Scientific Literature
SPECIAL BOOKS.
In these two handsome volumes[1] the distinguished Director General of the Geological Survey of Great Britain and Ireland traverses ground once hot with subterranean fires but long since cooled, and traverses, too, a field in science formerly heated by the fires of controversy which have burned themselves out like the ancient volcanoes. Werner's "geognosts," or the Neptunists, as they were dubbed, who in the latter part of the eighteenth century maintained the aqueous origin of basalt, were converted or silenced in the early part of the nineteenth and have left no followers. There are no more Plutonists, moreover, for all geologists are of that stripe. The author has designed this work as a summary of what has now been ascertained regarding the former volcanoes of the British Isles. "The subject," he says, "has occupied much of my time and thought all through life. Born among the crags that mark the sites of some of these volcanoes, I was led in my boyhood to interest myself in their structure and history. The fascination which they then exercised has lasted till now, impelling me to make myself acquainted with the volcanic records all over our islands, and to travel into the volcanic regions of Europe and western America for the purpose of gaining clearer conceptions of the phenomena." The British Isles afford a large and varied body of evidence regarding the progress of volcanic energy in former ages. The geological record is remarkably complete in those islands, and has been very carefully studied. The position of Britain, on the margin of a great ocean basin, is one in which volcanic action is apt to be most vigorous and continuous. Furthermore, denudation has made the extensive volcanic record of this 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.[2] Much of the popular interest which auroras have aroused is due to the varied and often changing forms that they assume. Their protean shapes have caused people of earlier times to see in them bloody flames, armies in the clash of battle, or furiously riding Valkyries. They may appear as faint lights without defined shape; if more distinct, they are seen to have the form of rays converging upon some point in the sky, or of more or less clearly defined arcs, or of hands which fold over on themselves like a curtain, and are called draped auroras. M. Angot describes many variations of these typical shapes, and presents plates on which some of the most interesting are depicted. The vertical rays of which most auroras seem to be made up move both horizontally and vertically, and as they are usually in constant motion an aurora may readily change from one typical form to another. While the light from most auroras is white, the rays are frequently tinged with yellow, and are sometimes red at the lower extremity and green at the upper. The nature of the auroral light is not established, although the spectroscope and polariscope indicate that it is emitted by luminous gases. It seems probable that a slight rustling or crackling sound accompanies auroral displays, but our author finds no credible evidence of any odor. While many auroras of small extent appear only as local phenomena in high latitudes, others are visible to within twenty degrees of the equator. It seems to be usual for an aurora australis to occur simultaneously with an aurora borealis, notably on February 4, 1872, when the globe, with the exception of an equatorial zone of about forty degrees, was enveloped in polar lights. The periodicity of auroras has been studied with the result of establishing a diurnal and an annual period, and a period of a little more than eleven years. Less exactly determined are the periods of about twenty-eight days, and of about fifty-five years and a half. Those of twenty-eight days and eleven years seem to connect the auroras with sun spots. The relations of the aurora with meteorological phenomena and with terrestrial magnetism have also been investigated. The data obtained from researches on the foregoing questions have given rise to many theories as to the cause of auroras. Our author states several hypotheses that have been made clearly untenable by recent advances of knowledge. One of these is the idea of Mairan that auroras occur when the earth passes through the cloud of matter that produces the zodiacal light, some of this matter falling into our atmosphere and becoming ignited. The reflection of sunlight from particles of ice in the atmosphere is another cause suggested, and still another regards the light as a sort of fluorescence. Our author treats with more respect, although positively rejecting it, the theory first definitely stated by Dalton, that the light is given off from silent electric discharges between the upper and lower strata of the atmosphere, these discharges being conducted through ferruginous dust falling upon the earth from space. He gives also several electric theories, among which he regards that of Edlund as the most satisfactory. Edlund starts from the phenomena of unipolar induction—the production of currents in a metallic sheath surrounding a magnet when the sheath is rapidly revolved. The general phenomena of terrestrial magnetism justify regarding the earth as such a sheath. Electricity, according to this theory, is constantly being carried by molecules of air from the equator to the poles, where it accumulates and from time to time returns to the ground by slow discharges which produce auroras. M. Angot concludes with a statement of the plausible but undemonstrable theory recently proposed by Unterweger. To the body of the work is appended a Catalogue of the Auroras seen in Europe below latitude 55° from 1700 to 1890, filling eighty-eight pages. A large part of the data employed in this volume were gathered in Lapland during the winter of 1838–'39 by the expedition on board the Recherche, others are taken from Nordenskiöld's Voyage of the Vega, and still others from a variety of sources which the author indicates. The book will go far toward giving the general reader clear ideas in place of fragmentary notions.