"Vol. 55.] INTO DIABASE AT SOREL POINT (nOHTHERN JERSEY). 445' 



was produced, and he concludes from this that the formation of the^ 

 mica is in relation to the acidity of the more central part where these- 

 fragments are found. It is also interesting to note that the- 

 granophyre contains a little biotite on approaching the gabbro/ due 

 to the fact that portions of the highly basic gabbro have been 

 incorporated with the acid magma near the junction of the two 

 rocks. Biotite is also one of the products of the alteration of 

 diallage at Barnavave. Mr. T. H. Holland, in his paper on ' Augite- 

 diorites with Micropegmatite from Southern India,' ^ has remarked on 

 secondary ' opaque-iron ores, hornblende and biotite,' situated on the 

 margins of the augite which abut directly against the micropegma- 

 tite. This instance is, however, scarcely comparable with those of 

 the Jersey rocks if, as Mr. Holland believes, the micropegmatite 

 represents the final product of consolidation as opposed to a later in- 

 jection of acid material — a difference which he carefully emphasizes.- 



The biotite in the biotite-gTieiss of Sark ^ is an excellent instance 

 of the production of this material by the second of the two methods, 

 closely resembling that followed in the rocks around Sorel Point. In 

 some cases it appears clear that a total reconstruction of a fragment 

 has taken place, with the formation of mica as a result of the union 

 of the hornblende of the diabase and the alumino-alkaline consti- 

 tuents of the granite. In the Jersey rocks the tendency to form 

 hornblende seems to have been very strong, for thin sections of the 

 basic rock, apparently not greatly impregnated with the acid magma, 

 show scarcely a trace of augite in the midst of the replacing horn- 

 blende; so that we can hardly expect the clusters of granular 

 augites mentioned by Prof. SoUas as one of the results of the 

 change in the diallage of the Irish rock. One instance only of 

 anything of this kind lias been seen (p. 442). There is, however, an 

 interesting likeness between one of the varieties of granophyre which 

 the same author has described/ namely, that containing biotite 

 and hornblende, and a specimen collected by the Hev. Edwin Hill m 

 Aldemey, and given by him to Prof. Bonney. 



Prof. SoUas, in describing the rock from Slieve Gallion, calls 

 special attention to the occasional presence of a diallage-core sur- 

 rounded by olive-green amphibole, and that again by an external 

 border of biotite. This characteristic, with others, leads him to 

 comment on the possibilities of distinguishing homogeneous rocks 

 from rocks of heterogeneous formation, even when the field-relations 

 are not known. Similarly, the augitic cores surrounded by horn- 

 blende and these again by mica, diagrammatically shown in the 

 Alderney rock, become especially interesting when looked at in the 

 light afforded by the rocks of the neighbouring island. 



I am indebted to the kindness of the Eev. Edwin Hill and Prof. 

 Bonney for permission to describe a thin section cut from this specimen. 

 In this the three minerals — augite, hornblende, and biotite — form 



^ Quart. Journ. G-eol. Soc. vol. h (1895) p. 133. 



^ Ibid. vol. mi (1897) pp. 413-414. 



3 Ibid. vol. xlyiii (1892) p. 131. 



* Trans. Roy, Irish Acad. vol. xxx (1894) p. 497. 



