AUTHOE'S PREFACE. xvil 



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 inter- 

 vals of repose throughout the remainder of Tertiary and Quaternary time, 

 the most recent ones having to all appearances taken place only a few cen- 

 turies "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 propylites), 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 pro- 

 digious 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 tlian 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 

 pubhcation of Richthofen's "Memoir on a Natural System of Volcanic 

 n p— ii 



