Cole — The VarioUte of Ceryg Owladys, Anglesey. 113 



Fulcher, who has kindly allowed me to use the whole of the 

 material which he collected. Our examination of the rock in the 

 field suggests certain comments on the earlier descriptions, and 

 some new points are arrived at from the study of microscopic 

 sections. 



The mass of variolitic diabase forms one of the little rocky 

 ridges and bosses which jut out above the sand- waste on the north 

 shore of Newborough Warren. The age of these rocks is Pre- 

 cambrian ; the whole series has become fissile through imperfect 

 cleavage, and has been in part broken up by earth-movement. 



The variolite is conspicuous by its characteristic greenish white 

 spherules, which stand out iipon the weathered surfaces above the 

 level of the dark and gray-green matrix. A common diameter for 

 these spherulites is 3 millimetres or less, and they nowhere approach, 

 in this exposure, the coarse proportions of those in many rocks of 

 Mont Grenevre. 



But the identity of this material with true variolite is clear to 

 anyone familiar with the types from the Durance. Despite the 

 partial foliation of the mass, and the consequent elongation of 

 many of the spherulites, the rock can be seen to bear the same 

 relation to the compacter diabase that the devitrified crusts of 

 glass do to the spheroidal rocks of Le Chenaillet or Mt. La Plane.^ 

 Thus Prof. Blake has described the variolite in Anglesey as 

 " running into the crevices and wrapping round the surface of a 

 purple calcareous rock ; " comparison with the masses of the 

 Western Alps shows that this appearance is due to its having 

 developed as a product of rapid cooling on the spheroidal surfaces 

 of a lava, the decomposition of the latter having produced the 

 " purple calcareous " material. 



Prof. Blake, in a letter to the present writer, has suggested 

 that the importance of the limestone may have been overestimated 

 in the field, and that this material may have arisen by chemical 

 changes in the igneous rock. This latter view I believe to be 

 undoubtedly correct, although as Prof. Blake suggests, the carbo- 

 nate of lime may have been in part imported by mineral springs.^ 



1 Q,uart. Journ. Geol. Soc, vol. xlvi., figs, on pp. 311 and 312. 

 - For notes on some of the limestones of this area, see the " Report on Eocks of 

 Anglesey," already cited, p. 23. 



