TJie Nomenclature of the Keratophyres. 353 



series of rock names new to the science, but they should be used 

 rather as textural qualifiers in conjunction with " keratophyre " 

 and " quartz-keratophyre ". Thus, to speak of an intrusive, 

 orthophyric keratophyre, in the writer's ophuon tells the whole 

 story — genetic relations, mineral contents, mode of occurrence, and 

 raicrostructure all being indicated. The three words could con- 

 ceivably he replaced by one, but the latter could never be self- 

 explanatory as the former statement is. 



VI. CoMP.\RisoN OF Keratophyre with Bostonite. 



The resemblance of some keratophyres to some bostonites in 

 mineral composition and texture has led to a certaiii. amount of 

 overlap})ing in the use of the two names, and to uncertainty con- 

 cerning the restriction of the usage of the terms. 1l\ illustration it 

 may be stated that Washington ^ labels the bostonitc of Marblehead 

 Neck " keratophyre (bo.stonite) ". The relative scarcity of dark 

 minerals, the predominance of alkali felspar, often strongly sodic, 

 the absence of fclspathoids and sodic " pyriboles " are features 

 shared by both rocks. In this country there is general agreement 

 in restricting " bostonite " to intrusive rocks, although this is not 

 the case iii the United States, and some doubt exists as to the actual 

 mode of occurrence of the type rock." Recently an attempt has been 

 made to widen the definition so as to include, in addition to 

 hypabyssal bostonites, those lavas [)0ssessing the same chemical 

 and mineralogical features. The suggestion was not accepted, 

 however, by the committee who recently reported on the Nomen- 

 clature of British Petrography.^ 



Considering, now, recent definitions of bostonite, the latest is 

 that of Holmes, who defines the rock in the Nomenclature of 

 Petrology as a leucocratic alkali syenite-aplite with tracfiytic texture 

 formed almost wholly of alkali felspars. In Hatch's descri^ition 

 of the same rocks, the kinds of alkali felspar are enumerated, com- 

 prising microcline, anorthoclase, perthite, and albite. The last is 

 included in the latest edition only.^ In this connexion it may be 

 noted that until recently the felspar of bostonites was regarded as 

 being sodi-potassic. Each of the analyses recorded in Washington's 

 Chemical Analyses of Igneous Rocks shows almost equal amounts 

 of soda and potash, the average of six analyses yielding NagOS'iS per 

 cent, and KgO •'^'38 per cent. It may be wondered, therefore, whether 

 the recent addition of albite to the above list may be due to the 

 confusion mentioned above of " keratophyre " with " bostonite ". 

 The felspar characteristic of, and commonly found in (intermediate), 

 keratophyres is albite. That characteristic of (intermediate) 



^ Washington, Chemical Analyses of Igneous Rocks, p. 146. 

 " Iddings, Igneous Bocks, vol. ii, p. 100. 



^ Report on British PetrograpJiic Nomenclature, 1921, p. 144 (see " trachyte "). 

 * Hatch, Text Book of Petrology, part i. Igneous Rocks, 1913, p. 239. 

 VOL. LIX.— NO. VIII. 23 



