350 A. K. Wells— 



our attention. Lime-hostonite (maenaite) was applied by Brogger to 

 certain leucocratic intrusives in the Gran district of South Norway.^ 

 The same name was chosen by Elsden for the chemically similar, 

 but genetically dissimilar felsite at Abercastle, in Pembrokeshire, 

 then regarded as a normal intrusive rock.^ At a later date this name 

 was applied to a similar, definitely intrusive rock in the Kilbride 

 Peninsula.^ Reynolds and Gardiner, in referring to their choice of 

 this name, remarked upon the striking similarity of the Kilbride 

 rock to the keratophyres of Skomer Island, but explain that they 

 preferred to use " lime-bostonite ", as " British avthors seem to 

 confine the name keratophyre to lavas ". During the re-examination 

 of the Abercastle rock, it became evident that the percentage of 

 normative anorthite had been considerably over-estimated, and 

 that the fels})ar of the rock was almost pure albite.* Thus, " lime- 

 bostonite " as applied to these rocks appears to be a misnomer. 



IV. Genetic Considerations. 



The descriptive terms mentioned above are undoubtedly useful ; 

 they indicate not only the mineral contents, but also the texture 

 of the rocks. But they fail to take account of the petro- 

 genesis. They do not suggest the genetic relationship of 

 these with other members of the Spilitic Suite. But genetic 

 considerations must not be ignored. There is little doubt that upon 

 the latter will be built up the classifications of the future. Thus 

 Harker quite definitely states that ..." a natural classification must 

 have regard to the mutual associations of rocks as well as to their 

 descriptive characters.'"'^ He evidently regards as the ideal a scheme 

 of classification based on the genetic relationships of different 

 rocks. Various authors, since the writing of the Natural History of 

 Igneous Rocks, have duly emphasized the same fact, none more 

 emphatically than Crook,® whose most significant phrases are quoted 

 and amplified by Holmes in the opening chapter of Petrographic 

 Methods. This ideal scheme is admittedly unattainable in the present 

 state of our knowledge, but a grouping togethei- of rocks genetically 

 related must be regarded as a step in the right direction, while 

 the inclusion under one name of rocks of difierent parentage is a 

 retrograde movement, and one likely to cause confusion. 



Dewey and Flett tentatively suggested the fact, afterwards 

 definitely stated by Cox, that keratophyric rocks, intrusive as well 

 as extrusive, belong to the Spilitic Sidte.' The constant association 



1 Brogger, Quart. Journ. Geol. Soc, vol. 1, 1894, p. 23. 

 ^ Elsden, Quart. Journ. Geol. Soc, vol. Ixi, 1905, p. 594. 

 ^ Reynolds & Gardiner, Quart. Journ. Geol. Soc, vol. Ixviii, 1912, p. 92. 

 * Cox, Quart. Journ. Geol. Soc, vol. Ixxi, 1915, pp. 312-14. 

 5 Harker, Nat. Hist. Igneous Roclcs, 1909, p. 369. 

 8 Crook, Min. Mag., vol. xvii, 1914, p. 72. 



'' Cox, Rep. Brit. Assoc. Birmingham, 1913, p. 496 ; and Dewey & Flett, 

 Geol. Mag., 1911, p. 207. 



