306 H. Stanley Jevons — Nomenclature of Igneous Rocks. 



the system of nomenclature here proposed. Petrologists of any 

 experience know more or less accurately from constant use the 

 index minerals of every family, and ambiguity is likely to arise 

 only in the case of rocks falling near the boundary between two 

 families. 



IV. The Prefixes a- and mono-. 



The recognition of index minerals and the formation of definitions 

 would be of little use, however, if the fact were not acknowledged 

 that rocks exist which do not contain the index minerals of any 

 family, and so cannot be classified at all if the scheme be rigidly 

 maintained. So far as I know, such aberrant forms are nearly 

 always schlieren, products of differentiation in immediate continuity 

 with and passing gradually into the parent mass ; and they are 

 therefore generally classed along with the rock of their parent mass, 

 from which they diff'er only in the absence of one of the index 

 constituents.^ To indicate the abnormality of their composition in 

 their names, I propose to place in the prefix the name of the absent 

 index mineral, preceded by the privative a-. This combined prefix 

 should stand immediately in front of all the other mineral prefixes, 

 separated from them by a hyphen. As examples of names thus 

 produced I may quote apyr-gabbro for anorthosite, which is essentially 

 a gabbro without the pyroxene, and apyr-magne-peridotite, which, 

 denotes certain of the schlieren which sometimes occur in masses 

 of gabbro and peridotite, and consist of magnetite associated with 

 olivine and only traces of other minerals. 



The difficulty of determining whether a mineral is actually 

 present or not, when it is suspected in small quantities, will tend 

 to produce confusion, if an index mineral must necessarily be 

 absolutely wanting before it can be declared absent in the name. 

 For all practical purposes an index mineral has vanished if its 

 proportion be less than 5 per cent, by weight of the whole rock, 

 and I therefore propose this amount as the limit, falling below which 

 the rock either passes into another family or requires the missing 

 index mineral placed in the prefix with the privative a-. 



There remains yet one more prefix whose use I have found 

 indispensable, namely mono- or mon-, meaning only. It is of use 

 chiefly in the case of sub-groups which are distinguished from others 

 of the same family by the absence of minerals, rather than their 

 presence. For instance, a rock which consists of nothing but the 

 index minerals of some family can only be distinguished from other 

 members of the same family by the absence of the minerals which 

 characterise them. This want of additional minerals may be 

 expressed by using mono- as the sole prefix in naming a rock. 

 Thus a mono-gabbro is a rock consisting solely of an intermediate 

 or basic plagioclase and a monoclinic pyroxene, the index minerals 



^ The word schliere is here used in the sense defined by Rosenbusch (" Elements 

 der Gesteinslehre," 1898, p. 41), Reyer (" Theoretische Geologie," p. 81), and Zirkel 

 (*' Petrographie," 1893, i, p. 787). It has been introduced into the English language 

 by Holland ("The Charnockite Series " : Mem. Geol. Siu-v. India, vol. xxviii, p. 217). 



