446 MR. J. PAKKINSON ON AN INTRUSION OF GRANITE [Aug. 1 899, 



small groups or nodes in a rock whose principal constituents for the 

 rest of the section are quartz and felspar. The fibrous augite- cores, 

 identical with the fragments of the same mineral described on p. 442, 

 are surrounded and penetrated by patches of dark green hornblende, 

 with which a dark brown and straw-coloured biotite is most inti- 

 mately mingled. Grains and dust of iron-oxide, and rarely pyrites, 

 are scattered through these nodes. The biotite, which occurs in 

 some profusion and often in large plates through the rest of the 

 section, together with decayed and decaying augites, is very late in 

 formation and extremely irregular and tattered in outline. There are 

 a few colourless crystals of a mineral resembling that described as 

 sillimanite on p. 442, and occasionally a zircon. Corroded felspars, 

 striped and unstriped, often with a peripheral zoning, lie in a 

 granular aggregate of felspar and quartz. The latter is abundant, 

 though any single grain seldom reaches large dimensions, nor does the 

 mineral preserve its optical continuity over a comparatively large area 

 as in some slides from Jersey. This mixture of quartz and felspar 

 in irregular and subangular grains is not the least peculiar feature 

 of this unusual type of rock. Mr. Hill's remark, after describing 

 sundry appearances in the field, that ' it is surely not necessary to 

 assume that every coarsely crystalline mass is throughout of simul- 

 taneous date,'^ seems to me in all probability to suggest the true origin 

 of the rock. The evidence of the production of biotite from the 

 replacing hornblende is, I think, conclusive ; and it is of great 

 interest to find the double change of the pyroxenic mineral exhibited 

 so clearly in this slide. In Jersey, the augite appears to be less 

 stable and the production of biotite from hornblende not to coexist 

 with the earlier change of augite into hornblende, or at all events 

 not to the same extent. 



Prof. Sollas has found in the rocks of Barnavave that ' in 

 junction-specimens uralitization is met with, but in xenocrysts this 

 has not been observed ; in their case the diallage directly passes into 

 a sage-green hornblende.' ^ In Jersey the brown hornblende, which 

 forms so readily from the augite of the diabase, is not met with in 

 the reconstituted rocks, nor is the actinolitic fringe, found here and 

 there in connexion with the ophitic plates of the former, seen with 

 the semi-porphyritic or granular green hornblende characterizing the 

 latter. 



It was mentioned on an earlier page that Noury has recorded 

 more than one intrusion of granite into the so-called diorite. That 

 from the south-east of the island has still to be described in detail. 

 It possesses many features in common with the intrusion so admirably 

 shown in the cliff's round Sorel Point. But the field-evidence is 

 rather more obscure, and the study of thin sections of the rocks 

 has revealed sundry points which render it advisable to postj)one a 

 description of the district to another occasion. In the more northerly 



1 Quart. Journ. Geol. Soc. vol. xlv (1889) p. 381. 



2 Trans. Eoy. Irish Acad. vol. xxx (1894) p. 494. 



