518 Scientific Proceedings^ Royal Dublin Society, 



rise to spherulitic tachylyte some 15 mm. thick, and alteration has 

 produced a variolite on a miniature sqale. The largest spherulites, 

 as seen in. section, are 1 mm. in diameter ; clear yellow-green 

 glass remains between them in places, but for the most part it has 

 become devitrified by alteration. Towards the interior, the sphe- 

 rulites are smaller, but in contact; they are built up of curving 

 fibres like their representatives at Mont Q-enevre, and the usual 

 poorly-defined dark cross is seen with polarized light. 



From the above considerations, and from the Anglesey example, 

 it is clear that variolite may be the product of an oli vine-basalt. 

 I have also recently noticed pseudomorphs after olivine in a 

 specimen of variolite picked up near Cesana Torinese, on the 

 Italian side of Mont Grenevre. The basic character of the rock 

 of Annalong has been shown by a partial analysis, which has been 

 kindly made for me by Mr. Basil L. Dunne, A.R. C.S.I., A.I.O. 

 The results were obtained through the kindness of Professor W. N. 

 Hartley, F.R.S., in the chemical laboratory of the Royal College 

 of Science for Ireland. But the considerable alteration of the 

 rock renders the figures unsatisfactory for purposes of comparison, 

 the carbonic anhydride being as much as 6'3 per cent. The 

 ferric oxide is 13'02, and the silica 46-89 per cent., that of the 

 variolite of the Durance, determined by Delesse,^ being 52'79. 

 Analyses of other basic dykes of the Mourne district have been 

 published by Dr. Samuel Haughton.^ 



Incidentally, during the observation of a special feature among 

 the dykes of Mourne, a general question is brought before us. 

 The condition of the selvages of many of these dykes, exposed as 

 they are to all manner of natural attacks, is such as to suggest 

 that they are not of very remote geological age. Although cut off 

 by the great granite intrusion, or traversed by its offshoots of eurite, 

 they may not be so far anterior in origin to these as has sometimes 

 been supposed. May they not represent the first fissuring and 

 ''starring" of this locality in earliest Eocene times, followed by the 

 granite of the Mourne Mountains, which in its turn yielded to those 



1 " Sur la Variolite de la Durance," Bull. Soc. Geol. de France, 2me serie, t. vii., 

 p. 430. 



2 " On the trap dykes of the district of Mourne," Journ. Eoy. Geol. Soc. Ireland, 

 vol. iv., pp. 95 et seq. 



