352 A. K. Wells— 



factor wliich can be so used is micro- structure, which will be con- 

 sidered later. 



The occurrence of free quartz in notable amounts in the most 

 acid keratophyres has created the need for distinguishing 

 " keratophyres " (intermediate) from " quartz-keratophyres " (acid). 

 The only synonym by which the latter name has been seriously 

 challenged is " soda rhyolite ". But it should be noted that this is 

 a wider, more embracing term than " quartz-keratophyre ", as it 

 covers not only the latter, but also the acid sodic lavas of the Atlantic 

 Suite. The argument formulated against the use of " soda granite- 

 porphyry " is equally applicable to " soda rhyolite " (and " soda 

 trachyte " also). The two primary groups into which the acid lavas 

 should be divided are (a) potash rhyolites, and (b) soda rhyolites. 

 The latter comprises two sub-groups : (i) soda rhyolites of the 

 Atlantic Suite, and (ii) soda rhyolites of the Spilitic Suite. Sub- 

 group (i) includes comendites and the more acid pantellerites, 

 while (ii) is equivalent to quartz-keratophyres. 



It has been suggested by some and believed by many others that 

 the keratophyres are merely the ancient and altered representatives 

 of the comparatively modern and fresh pantellerites and comendites. 

 Tf such were the case there would be no more justification for the 

 retention of " keratophyre " than there is for the use of " diabase " 

 for altered dolerites, or " propylite " for altered andesite. But 

 such does not appear to be the case. Miaeralogically and chemically 

 the two groups are distinct. The relatively low percentage of 

 alumina, high iron oxides and soda have resulted in the formation 

 of sodic amphiboles and pyroxenes in pantellerites, v/hile in 

 keratophyres for the same silica percentage the proportion of alumina 

 is higher and iron oxides much lower, so that non-sodic coloured 

 minerals were formed. These most acid niembers of the Spilitic 

 Suite are distinct not only mineral ogically and chemically, but 

 also genetically, from the acid Atlantic lavas, and their nomenclature 

 should indicate the fact. It is suggested, therefore, that 

 " keratophyre " should be restricted to the intermediate members of 

 the Spilitic Suite, both intrusive and extrusive, the rocks being 

 characterized by a high T)ercentage of sodic felspar, usually an acid 

 plagioclase near albite in composition, accompanied by a small 

 proportion of dark miiaerals, now represented by chloritic or 

 serpentinous pseudomorphs. Quartz-keratophyres are the corre- 

 sponding acid rocks, carrying in addition notable amounts of free 

 quartz. Both are apparently free from felspathoid muierals. Any 

 further subdivision can be carried out on the basis of textural 

 differences. There is little doubt that all the textural variations 

 exhibited by the fresh acid and intermediate lavas of other suites 

 will in time be discovered in keratophyric rocks. The following 

 textures have already been noted : cryptocrystalline, micro- 

 crystalline, variolitic, felsitic, trachytic, almost pilotaxitic, and 

 orthophyric. There is no need to disguise these difierences under a 



