292 PROF. T. G. BONNET AND MISS C. A. RAISIN ON [May 1899^- 



a separate boss, the question of its relation to the general mass of 

 serpentine must often be left undecided, for specimens occur which 

 mineralogically form transitions from both the diallage-rock and 

 the enstatite-rock to the serpentine. 



Actinolite-rocks. — These have been mentioned at many 

 localities, generally occurring where there is evidence of great 

 crushing. Sometimes the macroscopic aspect suggests a possible 

 relation to modified diallage-rocks. By microscopic examination 

 we can trace in more than one place a series showing an increasing 

 amount of alteration. 



(a) From south-east of Penrhyn-Fadog. — In slices of some speci- 

 mens from the enstatite-rock of the islet, a secondary actinolite 

 occurs. The latter mineral is occasionally scattered in fairly long 

 prisms showing a streaky and somewhat tafted grouping. In one 

 or two places it seems to be in connexion with granular patches of 

 augite. A pale greenish-grey rock, with remains of greenish crystals, 

 comes from the low reef immediately to the north. Microscopic 

 examination shows this to consist of fragments of diallage-crystals 

 interspersed with matted actinolite. A rock adjoining this is 

 markedly schistose, with thin crumpled whitish laminae in a dull 

 grey mass ; but the microscopic constituents are similar, though 

 more altered. 



(6) From the beach south of Fadog. — Three rocks sliced for 

 microscopic examination probably represent an irregular outcrop of 

 a mass or dyke, which doubtless was originally some form of 

 pyroxenite. One of these specimens is crushed and crumpled, 

 white and glittering with small crowded crystals. Microscopic 

 examination shows the ruins of crystals in bad preservation, appa- 

 rently diallage, shattered and sometimes split, with long narrow 

 actinolite-crystals scattered about. Between all these we find as a 

 groundmass, sometimes a tangle of small actinolites, sometimes 

 a clear colourless mineral, with one strongly marked cleavage, and 

 bright polarization-tints, the extinction-angle being too high for a 

 hornblende ; occasionally it is twinned : we regard it as a pyroxene. 

 It is, then, highly probable that this specimen represents a recon- 

 struction of a diallage-rock. 



The three following specimens are white or greyish and 

 crumpled : — (c) From Plas-coch Quarry, forming vein- or dyke- 

 like bands, described on p. 297; (d) from fields north-east of 

 Plas-coch ; and (e) from the beach south of Penrhyn-Fadog, 

 apparently in veins branching from the dyke of diallage-rock 

 mentioned on p. 278. The microscope shows that these might 

 be called actinolite-schists, for they consist of a foliated mass of 

 actinolite-fibres much crumpled, and sometimes showing strain-slip 

 cleavage. Specimens d & e are seen to contain a few grains of 

 magnetite, and the latter slice is slightly interstreaked with ser- 

 pentine. We consider these specimens to be an extreme product of 

 diallage-rock under pressure-metamorphism. 



