Vol. 55.] SEEPENTINE AND ASSOCIATED EOCKS IN ANGLESEY. 289 



Thus in these rocks the large pyroxene is mainly monoclinic, 

 while in the boulders it is chiefly rhombic, yet it seems probable 

 that the latter came from the yicinity of the former/ 



Many bosses or patches of rock, as previously mentioned, give a 

 slight suggestion of spherulitic structure, which, however, does not 

 become more definite on examination with the microscope ; still, 

 this may denote an incipient variolitic formation. 



The occurrence of radial nodules in serpentine has been described 

 by Becke, and attributed by him to the alteration of oli vine-cry stals." 

 We considered the possibility of this origin in the Anglesey ser- 

 pentine, but deemed it improbable because of the directions of what 

 seem to be cleavages, and the nature of the fibres. As a further 

 test, we etched an uncovered slide with hydrochloric acid. After 

 applying malachite-green, and washing, we found all the intervening 

 groundmass to be stained, while only a few lines of colour appeared 

 within the spherules; these probably mark some scattered and 

 included fibres of serpentine. Thus the original rock was most 

 likely a pyroxene-peridotite, saxonite or Iherzolite, the varioles 

 representing the pyroxenic constituent.^ 



III. ' Ophicalcite.' 



The term ' ophicalcite ' may be applied for convenience to various 

 rocks which consist partly of a carbonate of lime, partly of serpentine, 

 although they differ from the true ophicalcites, such as that of 

 Connemara. The formation of these ' ophicalcites ' has been de- 

 scribed by one of us '^ in the case of Cerig-moelion and some other 

 masses, and to these many outcrops are similar. They show^ 

 brecciated fragments of serpentine with calcite, infiltrated, and 

 deposited in veins.^ The carbonate sometimes forms definitely 

 shaped rhombohedra traversed by cleavage-planes, and penetrated 



^ This would mean a movement in a (roughly) north-easterly direction. At 

 any rate we know of no serpentine anywhere to the west, north, or east of these 

 boulders. The great northern ice-sheet, advocated by some geologists, must 

 have been taking a singular course in this district. 



2 Tscherm. Min. Petr. Mitth. ser. 2, vol. iv (1882) pp. 328-333, 355, & 450 ; 

 see also J. J. H. Teall, ' On Anthophyllite from the Lizard,' Min. Mag. vol. viii 

 (1889) p. 119. 



^ Dr. J. W. Gregory speaks of a serpentine of the Cottiau Alps as ' crowded 

 with needles and radial clumps of tremolite,' Quart. Journ. G-eol. Soc. vol. 1 

 (1894) p. 244. Some approach may be seen to the structure in slices of 

 Zoblitz serpentine in Prof. Bonuey's collection, and a very faint approach in 

 certain slices from Oarn Sparnaok and Laukidden (Lizard). 



* T. G, Bonney, Quart. Journ. Geol. Soc. vol. xxxvii (1881) p. 43, & 'Ligurian 

 & Tuscan Serpentines,' Geol. Mag. 1879, p. 366. 



^ Mr. Blake states that it is improbable that the Carboniferous Limestone 

 once extended over this district. Quart. Journ. Geol. Soc. vol. xliv (1888) p. 482. 

 But in the first place, infiltration from an overlying limestone into a crushed 

 serpentine is known to be the origin of some ophicalcites of this type. Secondly, 

 the former extension of the Carboniferous Limestone is probable ; for, although 

 the nearest point of it at the present day is about 7 miles distant, it is there in 

 force and is not likely to have died out so quickly. As older limestones in this 

 district are rare and local, it is less probable that they should have furnished 

 the calcite. 



Q. J. G. S. No. 218. u 



