OoLE — The VarioUte of Annalong, Co. Down. 515 



tachylyte, scarcely altered, and crowded with " cumulites " ^ and 

 other minute products of the primary devitrification (PL xxi. fig. 1). 

 A delicate banded structure, parallel to the wall of the dyke, is 

 noticeable on the very edg-e, and becomes rapidly more marked 

 as one proceeds inwards. Darker bands, formed of imperfect 

 spherulites, traverse the glass precisely as in the more familiar 

 rhyolites, and often wrap around the corroded felspars. Then 

 abundant minute spherulites, -05 mm. in diameter, replace the 

 cumulites in the ground itself ; and these aggregate into spheroidal 

 masses, brown by transmitted light, in a residual glass of some- 

 what lighter brown. This glass alters irregularly to vivid green 

 decomposition products. Grreatly elongated steam-vesicles, now 

 filled by zeolites and calcite, also occur freely. 



At 1 cm. from the edge, a somewhat abrupt transition occurs 

 from these patches of aggregated and minute spherulites to the 

 region in which the individual spherulites are distinctly visible 

 to the naked eye. Indeed, a plane of division exists between 

 the two regions, as if consolidation of the rock took place 

 through rapid chilling along the wall, while the main mass 

 remained fluid until an appreciable time had elapsed. Thus 

 the larger spherulites may be seen imperfectly developed, 

 starting from the zone of minute spherulites as from a base of 

 attachment that was already practically solid at the time of their 

 formation. They have, in fact, begun to grow from this surface, 

 as from the wall itself in the case of the tachylyte of Ardtun.- 

 Proceeding inwards, we find the normal type of the variolite (PL xxi. 

 fig. 2). Eod-like crystallites, straight or curved, abound in the 

 pale brown groundmass, from which slow secondary action has 

 removed much of the original glass. The spherulites, and the 

 cloudy matter round them, have developed from this groundmass, 

 as in the more highly silicated rocks, without disturbance or 

 obliteration of the first-formed crystallites. Rifts and cracks 

 occur through the rock ; but it is far less injured in this way than 

 is the case at Ceryg Gwladys, Anglesey. 



In the handsomely developed mass already referred to as 

 forming the S.E. end of the exposure, the once glassy ground 

 has become converted into a bright green chlorite. A few large 



1 Vogelsang, " Die Krystalliten." 



2 Quart. Journ. Geol. Soc, vol. xliv., pi. xi., fig. 1. 



SCIBN. PEOC. E,.D.S., VOL. VII., PAKT V. 2 S 



