248 SCIENCE PROGRESS 



multiplicity of series, it is better to make the percentage 

 of silica the means of dividing the granodiorites from the 

 diorites, and to call no rock with less than 66 per cent, of silica 

 a granodiorite. 



In using the name adamellite for the most acid division 

 of the monzonite series corresponding in respect to acidity 

 to the granodiorites of the calc-alkali series, and to the potash- 

 and soda-granites of the alkali series, I am following Brogger's 1 

 suggestion. Used thus it includes many of the rocks known as 

 biotite-granite, and hornblende-biotite-granite, which grade on 

 the one side into the granodiorites and on the other into the 

 alkali-granites, according as lime-bearing plagioclase or alkali- 

 felspar predominates. For practical purposes Lindgren's 2 useful 

 suggestion may, with a small modification, be adopted : namely, 

 that a rock in which less than one-third of the felspar is 

 orthoclase should be classed with the granodiorites (or with 

 the diorites when the silica is less than 66 per cent.) and one 

 with more than two-thirds of orthoclase with the granites (or 

 syenites, when the silica is less than 66 per cent.). Only 

 rocks in which between one-third and two-thirds of the total 

 felspar is orthoclase would be referable to the adamellite (or 

 monzonite) family. 



For the basic division of the monzonite series, I have 

 adopted Messrs. Hill and Kynaston's 3 term kentallenite, instead 

 of the more cumbersome synonym olivine-monzonite. 



The comprehensive name " alkali-gabbro," to include the 

 most basic divisions of the alkali series, was suggested to me by 

 Mr. R. H. Rastall, 4 of Christ's College, Cambridge. I have 

 extended it to cover kentallenite, on account of the relation 

 of that type to the shonkinites and essexites. In the shonkinites, 

 orthoclase is the dominant felspar; in the essexites, a lime- 

 bearing plagioclase (oligoclase or andesine) predominates, and 



1 Die Triadische Eruptionsfolge bei Predazzo, loc. cit. Christiania, 1895, P- 0I - 

 The name adamellite was used many years ago by Cathrein for the rocks now 

 known as tonalite ; but the name never came into general use, and there is, 

 therefore, no valid objection to its being used in Brogger's sense. 



* W. Lindgren, Amer. Journ. of Sci. 1900, p. 269. 



3 J. B. Hill and H. Kynaston, "On Kentallenite and its relation to other 

 Igneous Rocks in Argyleshire," Q.J.G.S., vol. lvi. 1900, p. 531. 



4 I am also much indebted to Mr. Rastall for the kind and ready response 

 he has always made to my many calls on his intimate knowledge of petrological 

 literature. 



