290 PROF. T. G. BONNEY AND MISS C. A. RAISIN ON [May 1 899, 



by or intercrystallized with fibres, of actinolite(?) or serpentine, as 

 already described by us in certain varioles. These associated crystals 

 are possibly pseudomorphic after some mineral which contained 

 lime and magnesia, and it might be, as in the yarioles, a pyroxene. 

 Some slices also include remnants of crystals with one close cleavage, 

 and a straight extinction, apparently enstatite. Here the carbonate 

 seems to creep along the cleavage-planes, but it is often difficult to 

 say whether the change has been by replacement of a mineral 

 in situ or by simple infiltration from without. Such replacements 

 will cause the rock to become more or less a dolomitized serpentine,^ 

 and this might happen without any very conspicuous structural 

 change, 



A set of specimens from Pelin-wen, into the details of which it is 

 perhaps needless to enter, in part may be explained as above ; but 

 in two instances these appear to be actual limestones, probably 

 Carboniferous, slightly dolomitized, veined by a serpentinous mineral 

 which has every appearance of being secondarily deposited, for, 

 besides certain peculiarities of structure, it occurs sometimes in 

 cracks not thicker than a sheet of paper. 



lY. Other Intrusive Eocks. 



Of the rocks associated with the serpentine, the gabbro by its 

 distribution suggests an intrusive mass, while others can be seen to 

 cut the former as dykes or veins. Mineralogically, they are often 

 closely connected with it. 



(1) (Pyroxenites, etc.) Diallage-rock^ Enstatite-rock^ etc. 



Dia 11 age- rock. — We found this in small patches in connexion 

 with several masses of serpentine, and the junction for the most 

 part (especially in the least modified examples) is abrupt. Along 

 the northern shore of the large inlet south of Penrhyn-Fadog it 

 is visible for about 12 yards or more, in the narrow rocky footway 

 along the beach. It interrupts a normal-looking serpentine, and 

 can be traced along the base of the low cliff', where it forms 

 branching veins. That it is intrusive in the serpentine cannot be 

 doubted, and the succession is similar to that at Lendalfoot and 

 other places.^ The rock consists of large crystals (often 1| inch 

 long) with the usual brassy lustre and well-marked cleavages, 

 associated with sparse spots or small patches of a dull purplish 

 serpentine. Under the microscope these prove to be small well- 

 formed crystals or rounded grains enclosed in, and lustre-mottling 

 the diallage ; in some cases they are enstatite, in others more 

 probably olivine. 



^ A whitish band, like a dyke or vein, in a crushed serpentine east of 

 PwU-pillo consists of dolomite enclosing fragments of serpentine, and is 

 probably due to a crush acting on one of these dolomitic serpentines. 



2 T. G. Bonney, * Serpentine, etc. of Ayrshire,' Quart. Journ. Geol. Soc. 

 vol. xxxiv (1878) p. 779. 



