310 H. Stanley Jevons — Nomenclature of Igneous Rocks. 



names must be accepted to a great extent, however meaningles& 

 and awkward they may be. There is, however, one family-name 

 which I have felt the necessity of altering, as reference to the table 

 will show ; that is, the name nepheline- or elceolite-syenite. The 

 name was probably the best which could be given at the time the 

 family was established, pointing out as it did the relation which 

 these rocks were then supposed to hold to a well-known group, the 

 Syenites.^ In view of what we know at the present time, however, 

 concerning the Alkaline Series, the name is not only useless but 

 misleading. In no essential quality does the so-called nepheline- 

 syenite resemble a normal syenite. The silica percentage, which 

 ranges, roughly speaking, from 50 to 60, is that of the more basic 

 diorites and some acid gabbros, rather than that of the syenites. 

 As regards mineral composition also, the two are absolutely distinct. 

 The normal syenite consists of orthoclase and a ferroraagnesian 

 mineral, which are the index minerals of the family, together with 

 a plagioclase felspar not exceeding the orthoclase in amount, and 

 usually a small quantity of quartz. The nepheline-syenite, on the 

 other hand, contains alkali-felspar (either potash or soda pre- 

 dominating) and nepheline as index minerals, with which are 

 associated ferromagnesian minerals in small quantity and mostly 

 of an alkaline type, and frequently sodalite, leucite, nosean,. 

 cancrinite, etc., without plagioclase or quartz. Hence it is evident 

 that merely removing the nepheline from a nepheline-syenite would 

 not make it into an ordinary syenite, and the name has therefore 

 no justification. Since the family is a member of the Alkaline 

 Series, and at the same time occupies a middle place in the range 

 of silica percentage, I propose to substitute for nepheline-syenite 

 the descriptive name midalkalite. This is compounded of *mid,' 

 the abbreviated form of ' middle,' and ' alkali,' which signifies the 

 series to which the family belongs. The only alternative was the 

 form medalkalite, adopting the Latin word medius for * middle.' 

 The word sounds strange and uncouth, however, to English ears, 

 and conveys its meaning much less clearly than midalkalite ; whilst 

 the latter is probably equally adaptable to French and German. 



Possibly the coining of a new name upon a new principle demands 

 some apology. It may be suggested that the better course would 

 have been to select one of the numerous names already given ta 

 various members of the nepheline-syenite family, and to extend its 

 meaning to embrace the whole group. Every other family-name 

 has arisen in that way, and a uniform system might have been 

 preserved. I consider, however, that there are very strong objections 

 to any such extension of the meaning of a name ; indeed, I hold that 

 an extension of meaning is justifiable only when the additional species 

 taken in do not surpass, either in the wideness of their difference from 

 one another or in number, the species already associated with the name 

 — the word species being here used merely to designate any small 



1 The name was given by Eosenbusch in 1877. He himself anticipates the- 

 objection which I am about to raise (see Mik. Phys., 1877, ii, p. 204), and. 

 proposes that, if it be insisted on, the n^axasfoyaite should be adopted instead. 



