this book will meet the need by creating a standard of reference which may to some extent limit future vacillations, and prevent unnecessary clashing between new terms. The main object of the book is, however, to be practically useful by serving as a guide to student, teacher, research-worker, and professional geologist, and indeed to all who need to follow petrological literature or to contribute to its pages.

The work involved in revising and amplifying the original card-catalogue, undertaken about a year ago, proved to be more arduous than was at first anticipated. It has, however, never descended to drudgery or mere compilation. On the contrary it has been in the nature of a literary exploration, leading one to examine a century's evolution of petrological thought and method, and to share the delights of many a curious traveller through little-known corners of lands the world. over. Unsatisfactory though certain parts of the nomenclature may be as an instrument of thought and exposition, it is, as a whole, unusually rich in pleasant associations, geographical, historical, and even psychological. Perhaps this romantic aspect of a subject bristling with technicalities is a dangerous one, for it tends to support the natural conservatism of even scientific men, and so, perhaps, to retard the development of that ideal system of nomenclature which we all hope for but cannot as yet create.

Certain attempts to systematise nomenclature have already been made, particularly in the field of

igneous rocks. It is desirable to draw attention to some of these, because, with a few exceptions, the terms they comprise have been excluded from the glossary that follows.

Jevons suggested a wholesale use of prefixes consisting of contracted, or rather mutilated, forms of structural, textural, and mineral names, together with a few chemical and qualifying syllables.' He thus arrived at such combinations as ophit-olidolerite, diopsi-mipegmo-rhyolite, rhomfels-pyralisyenite (= Laurvikite), and eudægi-midalkalite (=Lujaurite). Such proposals are obviously foredoomed to failure. In the words of Professor Bonney, "Time is not so valuable, or paper and printing so expensive, that we should talk or write ' gibberish' to save a few letters."

A more reasonable method has recently been proposed by Professor Shand, based in its application on the classification of igneous rocks according to his principle of saturation." He suggests—

(1) That the names of oversaturated and saturated rocks should end in the customary suffix -ite; e.g., granite, syenite, etc.;

(2) That the names of unsaturated rocks should carry

(a) the prefix sub-, or the suffix -ole, to indicate that the dyad or triad metals are unsaturated; e.g., subgabbro, for olivine-gabbro;

(b) the suffix -oid, to indicate that the monad

1 H. S. Jevons: Geol. Mag., 1901, p. 304.

2 S. J. Shand: Geol. Mag., 1917, p. 466.

3 Geol. Mag., 1913, p. 508; 1914, P. 485; 1917, p. 115.

metals are unsaturated; e.g., syenoid, for nepheline-syenite; and

(c) a combination of the prefix sub- and the suffix -oid, to indicate that both monad and dyad metals are unsaturated, e.g., subtheraloid or subgabbroid, for olivine-theralite. The principle adopted is excellent, but the choice of suffix, -oid, is unfortunate and cannot be accepted, for it has already been seriously overworked in other directions. It has been used in adjectival terms like granitoid and trachytoid, to express texture or composition; and in substantive form in the term pegmatoid, to denote very coarsegrained facies of igneous rocks differing from pegmatite proper by the absence of graphic-texture; and in terms of which dacitoid is a typical example, to connote similarity (to dacite) of chemical composition combined with dissimilarity of mineral composition.

The authors of the Quantitative Classification have introduced a very comprehensive nomenclature, the greater part of which is built up with the aid of a variety of suffixes and mnemonic contractions. Terms like felsic and mafic are extremely useful, and even though they have been regarded as technical slang, they have justified their invention by having been widely adopted. On the other hand, more ambitious and less useful terms such as alferfemphyric are ugly and have not met with a similar measure of favour. Cross, Iddings, Pirsson and Washington' have themselves made

1 These authors are referred to in the glossary by C.I.P.W,

the recognition of much of their nomenclature a necessity, for they have forcibly and persistently made use of it in a long series of publications which other petrologists cannot afford to ignore. Most of the new terminology, however, is intimately related to, and only used in connection with, the Classification itself, and with the latter it must therefore stand or fall. Unfortunately, the principles on which the Classification is based leave the main problems of petrology untouched, fail to open out new fields of research, and therefore do not constitute a creative contribution to the subject they were intended to illuminate. From this point of view the apparently wide influence exerted by the Classification in recent years has been largely factitious. Nevertheless, it is only fair to add that the authors of the Classification have rendered very real services to petrology by promoting greater accuracy of description and analysis, and by introducing the conception of the norm, which provides an admirable method of recalculating, comparing, and interpreting rock-analyses.

Another systematic terminology to which reference must be made has been proposed and extensively used by Grabau in his Principles of Stratigraphy (1913). The terms are summarised on pp. 296-7 of that work, and constitute an attempt, laudable in principle, to provide a comprehensive nomenclature for sedimentary and associated rocks. By means of a number of prefixes representing chemical or mineral composition and agency of formation, compound terms are built up at will by

combining them with a series of grade designations: rudyte, corresponding to gravel, shingle, pebbles, etc.; arenyte, corresponding to sand; and lutyte, corresponding to mud or rock-flour. Thus anemoarenyte in ordinary terms would be described as æolian sand; hydrosilicirudyte as quartz-conglomerate; and pyrolutyte as volcanic ash or dust. The extent of departure from current nomenclature is unnecessarily wide, and it seems doubtful whether such innovations will ever be recognised by adoption. Grabau's use of the terms. exogenetic and endogenetic is particularly unfortunate and tends to confusion of thought.' He describes rocks as "exogenetic " when they have been formed by agents acting from without, that is, acting externally with respect to, and independently of, the finished rock, as in the case of loose detrital sediments. Other rocks, formed by agents acting from within, he describes as "endogenetic, this category including igneous rocks, saline deposits, and organic accumulations. The two contrasting terms, although they are applied to rocks, are thus made to be synonymous with allogenic and authigenic respectively, and as the latter terms lead to a far clearer realisation of the primary division proposed by Grabau there seems to be no reason for rejecting them. The sentence "A calcareous sandstone contains allogenic grains held together by an authigenic cement," gives an accurate statement of fact, whereas the classifica

1 Amer. Geol., xxxiii, 1904, p. 228.