| OCR Text |
Show ATTTHOE'S PREFACE. xvii The second and principal subject of investigation comprises volcanic phenomena, The High Plateaus are in chief part a great volcanic area, in which eruptions have occurred upon a grand scale. The period of activity has been a very long one, its initial epoch having been not far from the Middle Eocene; and the eruptions have occurred with probably long intervals of repose throughout the remainder of Tertiary and Quaternary time, the most recent ones having to all appearances taken place only a few centuries ago. The variety of eruptive products is exceedingly great, all of the commoner kinds from the very acid to the very basic groups being well represented. The preponderating masses are trachytic, but rhyolites, ande-sites (including propylifces), and basalts are found in great abundance. Perhaps the most striking masses were the accumulations of fragmental volcanic products-the beds of conglomerate and tufa, which occur in prodigious volume, especially in the central- and southern portions of the district. These proved to be extremely interesting, yielding many themes of inquiry and speculation. It would have been impossible, under the circumstances, to apply to a region so extensive, so varied, and so ancient, the exhaustive analysis which Scrope has given to the volcanoes of the Auvergne or Geikie to the volcanic rocks of the Basin of the Forth. Of all geological, investigations the most difficult are those relating to volcanology. Where the accumulations are of great extent the student for a long time recognizes nothing but confusion, and the difficulty of evoking anything like order and a succession of events is about proportional to the amount of extravasation. And where the atmospheric forces have through long periods been at work destroying the piles which have been built up by eruption, the difficulty is still further augmented. Individual facts, indeed, are numerous and even bewildering by their number and variety. But we want something more than facts; we want their order, their relations, and their meaning; and it is rare to find the facts and relations so displayed that they are readily discerned and comprehended. It seemed best, therefore, to limit the inquiry to a very few questions. The one which was regarded with the most interest had reference to the Order of Succession of Volcanic Eruptions. Since the publication of Eichthofen's "Memoir on a Natural System of Volcanic h p-ii |
