294 John Parkinson — Gahbro of Pegli, N. Italy. 



Professor Bonney considers the probability that it had once been 

 a dolerite, perhaps ophitic, the glaucophane lesulting from the 

 " molecular rearrangement of the constituents of an aluminous augite 

 and labradorite." 



Description. — The rock at Pegli is exposed at the western end of 

 the "village, on the beach and under the houses built out seawards 

 from the left of the road. Macroscopically it is a dark rock, very 

 tough, and usually without conspicuous felspars, thongh occasionally 

 their remains give the rock the common black and white mottled 

 look of an ordinary fairly coarse gabbro. The ferromagnesian 

 constituent is much developed in the majority of cases, and in 

 this the slaty-blue tinge of the glaucophane is conspicuous. In 

 sections it is seen to be a diallage felspar rock usually altered 

 fliroughout by secondary changes, and showing much glaucophane. 

 In almost all cases it is clear that tlie diallage has contributed largely 

 to the formation of the glaucophane, and the term glaucophane gabbro 

 is therefore entii'ely applicable. The more usual change results in 

 a well-marked rim to the altering diallage, accompanied by more or 

 less definite flakes and scales of the same mineral in the body of the 

 parent ci-ystal. In some cases the flakes are grouped irregularly 

 without semblance of orientation ; usually, however, the relation is a 

 definite one, flakes and films of glaucophane are scattered throughout 

 the parent crystal, irregularly outlined and blotchily coloured, 

 but replacing the diallage in the same crystallographic sense. In 

 a few instances the diallage is changing into both glaucophane and 

 a brown hornblende ; when this takes place the two replacing 

 minerals are intimately associated together. This arrangement of 

 glaucophane and hornblende replacing diallage very closely resembles 

 that described and figured by Lacroix in an euphotide from 

 Villarodin.^ The absorption along c appears to me to be frequently 

 greater than in either the island of Syra or in the lie de Groix rocks, 

 for an examination of slides from which localities I am indebted to 

 the kindness of Professor Bonney. This is the varietur which 

 Lacroix terms "glaucophane foncee," characterized by the following 

 pleochroism : r, dark Prussian blue ; h, violet blue ; a, light greenish ; 

 yellow. This differs by its pleochroism from his " glaucophane 

 normale," which has c lavender blue, h purple blue, a colourless or 

 yellowish. In the Pegli rock the two are closely associated, and 

 replace one another indifi'erently ; the " glaucophane foncee " may be 

 either fibrous or platy in habit. 



The glaucophane rim usually found bordering the diallage is 

 nearly always more regular on its internal than on its external edge, 

 as though the formation of the replacing mineral had been continued 

 outwards as irregular root-like films and streaks into the surrounding 

 material, often extending for a considerable distance with a lighter 

 shade of blue. In some instances the irregular edge is practically 

 identical with the " flame-like forms " assumed by the blue amphibole 

 edging added to common hornblende which have been described 

 by W. Cross - from Colorado. By the production of flakes of 

 1 Miu. de la Fr. et de ses Colon., vol. i, p. 581, fig. 14. 

 ' Anier. Joiim. Sci., T.S., vol. xxxix (1890), p. 368, fig. 2. 



