lish equivalents have been omitted. The German appendix has been compiled by Miss J. H. Robertson, for whose help I wish here to acknowledge my gratitude. Appendices setting forth Greek and Latin words have also been added in order to make clear the significance of the many petrographical and mineralogical terms into which they enter. 66 Classification Tables have been introduced in order to bring together terms representing closely related concepts. These will be found useful not only in taking a broad survey of certain parts of the subject, but also in serving as a guide to new or forgotten terms. The tables also bring out the patchiness" of petrological nomenclature. Certain parts of the subject are heavily burdened with redundant terms, and with others that depend for their justification on differences so slight or trivial that they have but little practical value. Other parts of the subject, particularly those involving altered and metamorphic rocks have still a somewhat restricted nomenclature. This is probably due to the fact that until recent years attention had been focussed almost exclusively on igneous rocks, with the result that petrology has developed somewhat unequally. Fortunately the outlook is gradually widening, and even ore-deposits and meteorites are beginning to take a recognised place in the subject to which they properly belong. In the hope of making the glossary as representative as possible of customary modern usage, twenty preliminary proofs were pulled and sent to various petrologists with a view to eliciting suggestions, comments, and constructive criticism. This plan proved highly successful, and a very valuable series of replies was received from the following gentlemen Prof. T. G. Bonney, Prof. P. G. H. Boswell, Mr. Alfred Brammall, Prof. Grenville A. J. Cole, Prof. A. Hubert Cox, Mr. T. Crook, Dr. J. V. Elsden, Dr. J. W. Evans, Mr. J. F. N. Green, Dr. A. Harker, Dr. W. R. Jones, Prof. A. Lacroix, Dr. G. T. Prior, Prof. P. Quensel, Mr. W. Campbell Smith, Sir Jethro Teall, Dr. H. H. Thomas, Mr. G. W. Tyrrell and Prof. W. W. Watts. To all these I owe my heartiest thanks for their very substantial help in bringing to completion my self-appointed task. As a result of their approval of the project itself, and their active assistance in suggesting additions and modifications, I am supported in my belief that the book will meet a real need, and encouraged in my hope that the design originally conceived may have been brought to a successful issue. Finally, I wish to invite the further aid of those who use the glossary, by asking them to acquaint me with the particulars of any errors of interpretation or reference that may be detected, and to suggest any additional terms that have unwittingly been overlooked. Should the opportunity arise, such corrections will be incorporated in a revised edition, and as the "completion" of a glossary such as this is necessarily relative to the date of publication, authors of new terms are invited to ensure their future inclusion by communicating with me either directly, or, if necessary, through the publishers. ARTHUR HOLMES. Imperial College of Science and Technology, South Kensington, S. W.7, January, 1920. GLOSSARY A Aa, Dutton, 1883. A Hawaian term for block-lava, consisting generally of a rough tumultuous assemblage of clinker-like scoriaceous masses. = Aphrolithic lava. Cf. Pahoehoe. R. A. Daly: Igneous Rocks and their Origin, 1914, p. 291. J. A. Jaggar: Journ. Wash. Acad. Sci., vii, 1917. Aasby Diabase, Törnebohm, 1877.-A type of olivinedolerite containing biotite, ilmenite, and apatite, in addition to labradorite, augite and olivine. (Aasby, Sweden.) Absarokite, Iddings, 1895.-A porphyritic, basaltic or trachy-doleritic rock, characterised by the presence of phenocrysts of olivine, augite, and labradorite in a base containing orthoclase-mantled labradorite. (Absaroka Range, Yellowstone Park.) J. P. Iddings: Journ. Geol., iii, 1895, p. 938. Abyssal Assimilation, Daly.-See Assimilation. Accessory.-A term applied to minerals occurring in small quantities in a rock, and whose presence or absence does not affect its diagnosis. R. H. Rastall & W. H. Wilcockson: Q.7.G.S., lxxi, 1915, Accidental Inclusions, Harker, 1900.-A term applied to xenocrysts or xenoliths having no genetic connection with the igneous rocks in which they occur;= enclaves enallogènes. A. Harker: Journ. Geol., viii, 1900, p. 389. Achondrite, Cohen.-A general term for stony meteorites (aerolites) free from the spheroidal structures known as chondrules. Cf. Chondrite. Acid. A term applied to igneous rocks having a higher percentage of silica than orthoclase, the limiting figure commonly adopted being 66 per cent. Cf. Persilicic and Oversaturated. Adamellite, Cathrein, 1890.—A term applied originally to an orthoclase-bearing tonalite, and now used generally for granites in which plagioclase varies from one-third to two-thirds of the total felspar. = Quartz-monzonite. = (Mt. Adamello, Tyrol.) W. C. Brögger: Eruptivgest. Kristiania, ii, 1895, p. 61. Adinole, Haussmann, 1847.—A contact modification of shale or slate metamorphosed and albitised by doleritic (albite-diabase) intrusions; consists of a mosaic of albite, or albite and quartz, with interstitial chlorite and iron-ores. : H. Dewey Trans. Roy. Geol. Soc. Cornwall, xv, 1915, p. 71. Adobe, Russell, 1889.-A loess-like deposit occurring in the plains and basins of the Western States, and in the arid parts of Spanish America. Adsorption. A term applied to the change in concentration of solutions and colloids where they come into contact with surfaces. Aeolian.-A term applied to deposits whose constituents have been carried by, and laid down from, the wind. S. C. Stuntz & E. E. Free: U.S.A. Dept. Agriculture, Bureau of Soils, Bull. 68. Aerolite. A general term for meteoric stones, that is for meteorites composed mainly of silicates such as pyroxenes and olivine, with or without small quantities of nickel-iron, troilite, etc. According as |