THE NOMENCLATURE OF PETROLOGY INTRODUCTION. SOME years ago I began the compilation of a cardcatalogue of petrographic and associated terms, for the use of students in the Geological Department of the Imperial College of Science and Technology. Each card gave a brief description of the meaning (or meanings) of the term to which it was devoted, together with references to those papers on the subject which were available in the departmental library of the College, a library which, thanks to the collections made by the late Professor J. W. Judd and others, is unusually rich in author's separates. As the catalogue grew, its general usefulness became apparent; and a series of suggestions that it should be developed and published led me finally to the conclusion that such a course would not be unjustified. There are many geological glossaries in which petrological terms find a place, but they are for the most part old, and to-day they are but little used, or even known.' Several petrological books contain glossaries; notably Sir Jethro Teall's great work, British Petrography (1888), and J. R. Kemp's Handbook of Rocks. The latest edition of Kemp's book was published in 1918, and contains a wealth of information, particularly in relation to the older terms and the newer American terms. The only independent publication of the kind, however, appears to be the Lexique Pétrographique of Loewinson-Lessing (Paris, 1901). This invaluable work is now nearly twenty years old, and so luxuriant has been the growth of nomenclature during the last two decades, that the Lexique no longer serves as an adequate guide through the somewhat tangled forest of names. The complexity of petrological nomenclature at the present day is demonstrated by the following list, in which examples are given to illustrate the varying characters and principles on which names have been based from time to time. Classical basalt, basanite, obsidian, porphyry, syenite. Popular chert, cokeite, forellenstein, gabbro, gneiss, granite, greisen, hälleflinta, loess, marl, minette. Structure: augen-gneiss, banket, cipolino, dermolith, folia 1 Among those examined for the purpose of this book are the following: G. Roberts: An Etymological and Explanatory Dictionary of the Terms and Language of Geology, 1839. D. Page: Handbook of Geological Terms, 1859 & 1865. G. H. Kinahan: A Handy Book of Rock Names, 1873. T. H. Oldham: Geological Glossary, 1879. tion, knotenschiefer, lithophysæ, oolite, perlite, puddingstone, rhyolite, schist, variolite. Texture: anamesite, aphanite, granulite, hornstone, lithoidite, pegmatite, rhomb-porphyry. Roughness: grit, trachyte. Colour: eclogite, graywacké, greenstone, leucocratic, leuco- Organic characters: coral-sand, crinoidal limestone, diatomite, globigerina-ooze, lignite, miliolite. Mineral characters: aplite, diorite ; albitite, amphibolite, anorthosite, argillite, augitite, hornblendite, quartzite, peridotite; albite-enstatite rock, anorthite rock, muscovite-rutile rock, quartz-barytes rock; glauconitic sandstone, glaucophane-schist, hornblendegranite, mica-schist, nepheline-syenite, olivine-basalt, quartz-monzonite, sillimanite-gneiss. Chemical characters: alkali-rocks, anthracite, calc-alkalirocks, calc-flinta, calciphyre, picrite, soda-rhyolite. Use: laterite, novaculite. Mode of formation: crush-breccia, flow-breccia, mylonite, stalactite. Alteration: diabase, rapakivi. Relative age: palæopicrite, proterobase, protogine. Tribal names: gondite, ossypite. Surnames: buchnerite, charnockite, dolomite, grahamite, ulrichite. Place-names: cornubianite, ivernite, norite ; andesite, bostonite, canadite, jacupirangite, laurdalite, monchiquite, nevadite, sussexite, tonalite, wyomingite. Hunne diabase, Markle basalt, Ponza trachyte. Compound rock-names: granodiorite, rhyodacite, syeno. diorite, trachydolerite. Greek prefixes: apo-rhyolite, epidiorite, hyalobasalt, katagneiss, micropegmatite, orthogneiss, paragneiss, pseudotachylyte. Greek suffixes: basanitoid, dacitoid, graneid, pegmatoid, syenoid. Mnemonics: felsic, femic, mafic, salic. For many years the fashion has been established of basing new rock-names on geographical names, a method that burdens the memory with many ugly and cacophonous terms, leads sometimes to redundancy, and fails to suggest the distinctive characters of the rock-types so described. It is difficult, however, to see how these objections can be altogether avoided. A different application of the method has sometimes been made, a new type being described partly in terms of a well-known rock-name, and partly in terms of the locality where the type-rock occurs. Thus we have Ponza trachyte, Hunne diabase, and Markle basalt. More purely descriptive names, formed by adding mineral-prefixes to existing rock-names, such as biotite-hornblende-granite, are self-explanatory; and the same advantage is shared by compound terms like granodiorite, trachyandesite, and melanocratic olivine-trachydolerite. There is much to be said in favour of combinations of these kinds, as they reduce the number of fundamental names to be remembered, and are of wider application than specific names. Many protests have been made against the use of long compound-names, but, unless they become ridiculously cumbrous, they are thoroughly justified in the interests of clearness, as they are, for example, in organic chemistry. A geographical appellation already established, such as lugarite or marloesite, should not be |