312 H. Stanley Jevons — Nomenclature of Igneous Rocks. 



rocks. Guided by the principle above set forth, I have selected the 

 name of the soda rocks in each case, because they are far more 

 important, both in number and variety, than the corresponding 

 potash rocks. The family of Theralites and Shonlcinites therefore 

 becomes the family of Theralites, and the family of Ijolites and 

 Missourites becomes the family of Ijolites. The name ijolite is 

 neither convenient nor euphonious, but I fear there exists no 

 alternative which could be adopted. It is not objectionable enough 

 to warrant the proposal of a new name, and there is no other name 

 in the family which has sufficient extension to allow it to be applied 

 to the whole family in accordance with the above-mentioned rule. 



The chief reason for combining the soda and potash rocks in one 

 family as I have done, is that if they were separated here, amongst 

 the plutonic alkaline rocks, the same multiplication of families 

 ought to be allowed throughout the whole igneous series. Instead 

 of about thirty-five families, as we now have, we should then have 

 nearly sixty, each with a name to be remembered. This seems to 

 me too large a number, in the present state of our science. It will 

 be observed, however, that I propose still to keep the potash and 

 soda rocks separated, making them subfamilies instead of families, 

 and naming them accordingly. The theralites and shonkinites, for 

 instance, become respectively natr other alites and halitheralites. 



The silica percentages, which are stated for each family in 

 Table IV, are intended for information only. They show roughly 

 the limits of silica variation in each family, so far as its members 

 have been analysed and the results have been accessible to me, but 

 they must not be considered as a part of its definition. The limits 

 are stated in whole numbers for the sake of clearness, but they 

 should be read so that the limit stands 0'5 per cent, on the outside 

 of the figure given. A range of 67-57 per cent., for example, must 

 be read as 67*5 -56*5 per cent. This convention is necessary 

 because the limiting values fall more frequently on the half unit 

 than the unit. 



Table IV. Scheme of Classification of the Plutonic Division of the 

 Alkaline Series of Igneous Bocks. 



Family I.— ALIGRANITES. (= Alkaligranites, Ros.) 



Two Index Minerals : Quartz ; and an alkali-felspar. 

 SiOj % 78-68. 



1. Bi-aligranite (= Nordmarkite, Brogger, in part). 



2. Amph-aligranite {includes Riebeckite-granite, Saube). 



3. Pyr-aligranite. 



Family II.— ALISYENITES. (= Alkalisyenites, Eos.) 



One Index Mineral : An alkali -felspar. 

 Si O2 % 67-57. 



1. Pyrobiamph-alisyenite (= Pulaskite). 



2. Biamph-alisyenite (= Nordmarkite, iAe greaferjjarf). 



3. Monamph-alisyenite (= Umptekite). 



4. Rhomfels-pyr-alisyenite (= Laurvikite). 



5. -^g-alisyenite (= ^gyrine-syenite). 



