Transcriber Note Text emphasis denoted as _Italics_. THE International Scientific Series VOL. XXXV. _Frontispiece._ [Illustration] Sections of Igneous Rocks, illustrating the passage from the glassy to the crystalline structure. 1. Vitreous Rock. 2. Semi-Vitreous Rock. 3. Vitreous Rock with Sphærulites. 4. Rock with Crypto-crystalline Base. 5. Rock with Micro-crystalline Base. 6. Rock of Granite Structure built up entirely of Crystals. [_See pp._ 63-68. VOLCANOES WHAT THEY ARE and WHAT THEY TEACH BY JOHN W. JUDD, F.R.S. PROFESSOR OF GEOLOGY IN THE ROYAL SCHOOL OF MINES _WITH 96 ILLUSTRATIONS_ SIXTH EDITION LONDON KEGAN PAUL, TRENCH, TRÜBNER & CO. Ltd. PATERNOSTER HOUSE, CHARING CROSS ROAD 1903 (_The rights of translation and of reproduction are reserved._) PREFACE. In preparing this work, I have aimed at carrying out a design suggested to me by the late Mr. Poulett Scrope, the accomplishment of which has been unfortunately delayed, longer than I could have wished, by many pressing duties. Mr. Scrope's well-known works, 'Volcanoes' and 'The Geology and Extinct Volcanoes of Central France'--which passed through several editions in this country, and have been translated into the principal European languages--embody the results of much careful observation and acute reasoning upon the questions which the author made the study of his life. In the first of these works the phenomena of volcanic activity are described, and its causes discussed; in the second it is shown that much insight concerning these problems may be obtained by a study of the ruined and denuded relics of the volcanoes of former geological periods. The appearance of these works, in the years 1825 and 1827 respectively, did much to prepare the minds of the earlier cultivators of science for the reception of those doctrines of geological uniformity and continuity, which were shortly afterwards so ably advocated by Lyell in his 'Principles of Geology.' Since the date of the appearance of the last editions of Scrope's works, inquiry and speculation concerning the nature and origin of volcanoes have been alike active, and many of the problems which were discussed by him, now present themselves under aspects entirely new and different from those in which he was accustomed to regard them. No one was ever more ready to welcome original views or to submit to having long-cherished principles exposed to the ordeal of free criticism than was Scrope; and few men retained to so advanced an age the power of subjecting novel theories to the test of a rigorous comparison with ascertained facts.