146 THE INCLUSIONS OF THE VOLCANIC ROCKS 



crystals and rocks respectively without discussing the genetic origin, and these terms 

 have come into general use in English.* Harker in 1900 proposed to replace Lacroix's 

 terms by the terms " cognate xenoliths " and " accidental xenoliths " respectively, 

 on the ground that Lacroix relied on analogies of composition between inclusion and 

 host and not upon analogies of origin. | Holland the same year suggested the term 

 "autolith" in antithesis to "xenolith," rejecting Lacroix's term owing to a mistranslation 

 of "homoeogene" by "homogeneous. "J In the subsequent yearLacroix pointed out that 

 Harker' s criticism was based on a misunderstanding: 



" Le principe e nonce par M. Harker est precisement celui qui m'a guide dans 

 l'etablissement des deux groupes d'enclaves en question et dans leur nomenclature ? "§ 



Harker in 1909 repeated his objections in a modified form,|| but, in the writer's 

 opinion, has not succeeded in demonstrating the unsuitability of Lacroix's terms 

 or the superiority of his own. Lacroix's classification is therefore adopted in 

 this paper. 



Lacroix has further subdivided his group of homceogenous ^f inclusions into the 

 following groups : ** 



Allomorphe ...... homologue. 



antilogue. 

 Plesiomorphe ...... homologue. 



antilogue. 

 Polygene ....... exopolygene. 



endopolygene. 

 Pneumatogene. 



The allomorphous inclusions are those torn from masses already consolidated in 

 depth. The plesiomorphous are segregations formed in the magma that have not 

 had a separate existence as geological units. For these Holland's term of autolith 

 seems appropriate. Either of these two types may be homologous, i.e. formed by the 

 integral consolidation of the mean type of the enclosing magma, or antilogous, i.e. 

 formed by basic differentiation or original heterogeneity. The polygenous types are 

 those which have arisen by the action of the magma on included fragments of other 

 rocks, the endopolygenous resulting from a complete melting of the inclusion and the 

 crystallisation of the endometamorphosed portion of the magma, the exomorphous 

 from the transformation of the inclusion under the influence of emanations coming 

 from the magma. The latter type must therefore grade off into enallogenous inclusions. 

 Finally the pneumatogenous types arise in the way described above for the sanidinites 

 of Somma (vide ante). 



It is obvious that this classification is ideal and can only be applied in cases where 



* Sollas, W. J., " On the Volcanic District of Carlingford and Slieve Gallion," Part I ; " On 

 the Relations of the Granite to the Gabbro at Barnanave, Carlingford " ; Trans. Roy. Irish Acad., 

 xxx, 1904, p. 493. 



f Harker, A., " Igneous Rock Series and Mixed Igneous Rocks," Journ. of Geol., viii, 1900, 

 p. 309. 



% Holland, T. H, " The Charnockite Series ; a Group of Archaean Hypersthene Rocks in 

 Peninsular India," Mem. Geol. Surv. India, xxviii, Part II, 1900, p. 217. 



§ Lacroix, A., " Sur deux nouveaux groupes d'enclaves des roches eruptives," Bull. Soc. fr. 

 Min., xxiv, 1902, pp. 488-9. 



|| The Natural History of Igneous Rocks, pp. 346, 347, 1909. 



Tf Dr. Craigie, one of the collaborators on Murray's Oxford Dictionary, in conversation with 

 the writer in 1907 suggested " homceogenous " as the correct English translation of " homoeogene." 



** La montagne PeUe et ses eruptions, Paris, 1904. Also Bull. Soc. fr. Min., xxiv, 1901, 

 pp. 488-504. 



