Cole — The Variolite of Annalong, Co. Down. 513 



I owe several of the atove measures to Mr. Chute. Close 

 against the variolite on the north-east there crops up a dyke of 

 ordinary dark basalt, structureless to the eye, and branching into 

 a number of thin grey-green veins at the landward end. Micro- 

 scopic examination shows tliis to be a typical basalt rich in olivine, 

 with a thin selvage of translucent brown tachylyte. Traces of 

 glass remain on the contact-surfaces of many of the dykes of 

 LLourne ; and the suggestion occurs that in the variolite dyke of 

 Annalong we may have the very crown, or upward termination, of 

 the intruded rock, a portion that would be more free from crystals 

 and would cool far more rapidly than the lower mass. This 

 portion is lost to us in hundreds of other dykes, owing to the 

 extensive denudation of the coast since their intrusion. The 

 arrangement of the bands of spherulites (" varioles "), and the 

 appearance of the most landward mass of the dyke like a dome 

 from beneath the surrounding strata, certainly favour the view that 

 we are near the crest of the dyke. 



The Annalong variolite exhibits the structures of a once vitreous 

 mass in a more admirable degree than any other example that 

 I have examined. The coarse spheroidal pillow-like structure, 

 characteristic of the vast masses at Mont Grenevre and of the highly 

 altered rock of Ceryg Gwladys, is not present in this comparatively 

 thin dyke ; nor does the glass appear to be anywhere perlitic. But 

 the spherulites are remarkably beautiful and distinct, increasing in 

 size and closeness towards the centre, and also towards the seaward 

 end of the mass. In the latter part they are sometimes 1 cm. in 

 diameter, and the oxidized iron has coloured them a warm brown- 

 red, the outermost zone of each being a pale greenish- white. The 

 interstitial altered glass is dark green, and has now a hardness of 

 about 3-5, the spherulites being from 5 to 6. A polished surface 

 of this rock would be distinctly handsomer, for ornamental purposes,^ 

 than corresponding specimens from the Durance. 



In the centre of the greater part of the dyke the rock is purple- 

 black, and the closely-set spherulites are barely distinguishable 

 without the microscope. As one proceeds outwards, the increas- 

 ing proportion of devitrified glass gives a dark green tinge in 

 places, against which the spherulites appear greenish- white with 

 purplish centres. Banded structure, formed by coalescing spheru- 

 lites, is beautifully seen, and at length, some 7 cm. from the 



