Cole — llie VarioUte of Ceryg Givladys, Anglesey. 117 



cooling after the consolidation of the basic lava, and has finally- 

 resulted in the variolite. 



The rock, when molten, brought up with it porphyritic crystals, 

 which have been much corroded and penetrated by the ground-mass 

 round them (fig. 4). They are now altered to pale green pseudo- 

 morphs, in which yellow epidote has freely developed. In places 

 the epidote, by accumulation of granular crystals diversely 

 orientated, replaces the original porphyritic constituent, the 

 boundary of which is still preserved. 



The six-sided form of the sections of these porphyritic crystals 

 attracts attention, and in rarer cases evidence remains of irregular 

 cracks traversing them, along which magnetite has developed 

 (fig. 2). The sections have frequently four obtuse and two more 

 acute and opposite angles, as so commonly occurs in olivine. In 

 the latter mineral such sections arise by cutting the crystals 

 parallel either to the vertical axis or to the base. A section 

 parallel to the brachypinacoid will thus typically be bounded by 

 traces of the macropinacoids and the four macrodome planes, in 

 which case the acuter angles, formed by the macrodome, will be 

 76" 56', and the obtuser, where these planes meet the macropina- 

 coids, will be 141° 32'. A basal section may often show the traces 

 of the macropinacoids and of the four prism planes ; the two smaller 

 angles of the elongated hexagon will now be 85° 58', and the more 

 obtuse angles will be 137° V. In the case of the porpliyritic 

 crystals under examination, the most symmetrical sections being 

 selected, a number of measures gives the smaller angles as from 

 67° 30' to 90° 40', while the four obtuse angles measure from 

 134° 20' to 144° 25'. It will be seen that these results, allowing 

 for the frequent corrosion of the faces, and for obliquity in the 

 direction of the sections, agree very well with those obtainable 

 from olivine in microscopic preparations. 



Moreover, in one slide an elongated example of these crystals 

 occurs, reminding one strikingly of the olivine figured by Mr. 

 E. S. Dana^ from the basalts of the crater of Mauna Loa in the 

 Sandwich Islands. This crystal (fig. 3) may form another link, 

 however trifling, between the European variolites and the remark- 



^ " Contributions to the Petrography of the Sandwich Islands," Amer. Journ. of Sci., 

 vol. xxxvii. (1889), p. 446. 



