296 John Parkinson — Gahhro of Pegli, N. Italy. 



from the normal mineral by its higher extinction angle, and at 

 the same time hy a slight difference of pleochroism, consisting 

 principally in a greenish shade parallel to c. This he considers 

 intermediate to some extent between normal glaucophane and horn- 

 blendes poor in alkalis. Extinction angles high for glaucophane, up 

 to (say) 20° or even higher, are found in the Pegli rock. In many 

 instances the general appearance of the mineral and its connection 

 with definite remnants of diallage indicate that the high extinction 

 is due to its derivation from a pyroxene, but the pleochroism scarcely 

 agrees with that given by Lacroix. When the depth of coloration is 

 irregular the extinction is irregular also, and not well defined in 

 relation to the pleochroism. 



It seems perfectly reasonable to suppose that a local deficiency of 

 soda during the process of glaucophane manufacture would lead to 

 the formation of an actinolite at such a point, and that the withdrawal 

 of the same constituent from established glaucophane would produce 

 the commoner form of hornblende by an inverse process. The con- 

 siderable proportion of alumina which glaucophane usually contains 

 appears a difficulty to the understanding of this change to actinolite ; 

 but Foullon^ has published an analysis of a very fibrous soda-bearing 

 hornblende — practically identical with glaucophane — which he 

 terms Rhodusite, and in which the alumina percentage is about half 

 per cent. Unfortunately it is impossible to isolate the glaucophane 

 from the Pegli rock, and an analysis cannot therefore be prepared. 

 Judging from the literature of the subject, however, actinolite 

 appears to be by no means the only mineral into which glaucophane 

 may pass. Dr. Barrois, describing ^ the glaucophane rocks of the lie 

 de Groix, mentions the change of the characteristic constituent into 

 smaragdite ; the change into crocidolite has been discussed by 

 Koto as mentioned above ; Williams, in the paper already cited, 

 has described a possible change to arfvedsonite in pebbles from 

 Gesmagnano, and Columba^ has recently proved an alteration into 

 chlorite. 



The character of the groundmass has been already briefly 

 described by Professor Bonney. Its most remarkable point is the 

 absolute reconstitution of the original felspar, so that no hard and 

 fast line can be drawn between the altering diallage and the 

 material in which it is set. This varies greatly from place to 

 place. Prolongations of the glaucophane, detached scales and flakes 

 of the same mineral, associated with abundant similar flakes of a 

 rather dark-green and inert mineral, in many cases no doubt actino- 

 lite, though occasionally perhaps chlorite, lie scattered in a water- 

 clear substance, which resolves itself, under crossed nicols, into a 

 fine mosaic. Most of this is no doubt a secondary felspar, but it is 

 only in very rare instances that the characteristic twinning places this 

 beyond doubt. By diminution in the amount of actinolite and 

 glaucophane, and by the appearance of a rather dirty and granular 



1 Sitz. Akad. Wien, vol. 100 (1891), pp. 169-171. 



- Soc. Geol. du Nord, t. xi (1883-4), p. 50. 



3 Atti E. Accad. Sci. Torino, vol. xxix (1894), p. 416. 



