140 KOCKS OF ALDERX&Y. 



than of chloritisation. The biotite is sometimes reduced to 

 opaque masses of black magnetite. There are indications 

 that hornblende is present, basal sections showing fine cleav- 

 age and twinning; but other examples are mere skeletal re- 

 mains, chloritised, or altered to biotite. 



Indications of alteration are common in the felspar, the 

 zoned orthoclase showing turbid material surrounding clearer 

 centres, with clearer matter outside. In other instances there 

 are turbid centres and periphery, with clearer zones between. 

 Much of the felspar is completely decomposed to kaolin and 

 quartz . The orthoclase, unlike most of the potash felspar 

 of these islands, is white. 



Interesting indications of pyramidal quartz, surrounded 

 by allotriomorphic quartz, with strain, are associated with 

 the conversion of hornblende to biotite. Original iron ores 

 are notably absent. ...... 



South Crabby Granite. The rock shows a higher pro- 

 portion of felspar than is seen in the Bibette granite, and a 

 lower proportion of ferromagnesian minerals. The chlori- 

 tised and ferruginous remnants of the latter leave scarcely 

 sufficient evidence to indicate their original nature. They 

 are now shapeless and ragged masses bordering a clear 

 mineral. Iron-stained cracks of original cleavage indicate 

 decomposed hornblende, but biotite also- has been present. 



The felspar is more regularly kaolinised, the fragments 

 are smaller, and interlacing laths are more frequent. Much 

 of the orthoclase and quartz is distinctly granular. The 

 small amount of plagioclase present is albite-oligoclase. 

 There is evidence of kaolinisation, masking albite twinning, 

 which may be more common than is readily seen. 



Quartz appears to occupy the space belonging to the 

 original amphibole, bordered by patches, and traversed by 

 streaks of magnetite. The quartz forms irregular granules. 

 The rock is " granulitic " in the sense of the French writers. 



North Tourgis Granite. Quartz is not in such 

 abundance as in the north of Alderney, although where it 

 occurs, the patches are fairly large. It is clear, threaded 

 and traversed with strings and rows of inclusions, and shows 

 numerous curving cracks. These are filled with kaolin. 



Felspar is abundant. The kaolinisation has progressed 

 further in the orthoclase than in the plagioclase. The latter 

 is sometimes almost as clear as the quartz, kaolinisation 

 being restricted to the longitudinal cleavage planes, rather 

 assisting than masking the detection of poly synthetic twin- 

 ning. Oligoclase seems to predominate as the plagioclase, 

 although in some instances albite is suggested. The 



