Frof. Grenville Cole — The Tachylytes of Co. Down. 221 



not of feathery bunches of felspathic rods, but of " cryptocrystalline " 

 material with a delicate radial structure, thus resembling more 

 closely than usual the spherulites of rhyolitic rocks. Their colour 

 in reflected light is greyish- white, and in transmitted light grey- 

 brown and yellow-brown ; a certain amount of magnetite has 

 formed in dusty aggregations round about them, or along the lines 

 of junction of their component groups of fibres. As has been 

 observed in the spherulites in the tachylyte of Ardtun, the yellow- 

 brown material is distinctly pleochroic. 



Cracks traverse the spherulites freely, passing also across the 

 groundmass ; but I have found no trace of the " pseudo-crystals " 

 that are so prominent a feature in Continental variolites. The cracks 

 have originated from subsequent earth-movements, probably at the 

 time of the intrusion of the great granite masses of the Mourne 

 Mountains, which cut off so many of these coast-dykes abruptly.^ 



Some of the spherulites, however, have been broken up by the 

 fluid glass, which has entered in between the detached fragments 

 and has formed a microscopic network of intrusive veins. In several 

 cases an early phase of the rock is revealed to us by the occurrence 

 of crystals of plagioclase as nuclei to the spherulites. These crystals 

 have been greatly corroded by the magma round them, and many of 

 them must have been entirely absorbed. The glass intruded into 

 them is crowded with globulites ; but its present condition does not 

 enable us to judge of the character of the original unaltered magma. 



The altered glass forming the groundmass of the variolite is, as 

 usual, greenish ; in large part it is chloritized, but epidote has only 

 locally developed. A minute spherulitic and cumulitic structure 

 w^as originally present in it, and may be very generally traced. 

 Perlitic structure may have also occurred, but it is only obscurely 

 discernible in one of the six sections that I have studied. 



This rock gives no evidence as to whether it is the spherulitic 

 representative of a basaltic andesite or of an olivine basalt. I have 

 no hesitation in classing it as a variolite, and it is, I believe, the 

 fifth known representative of this rock in the British Isles, the 

 others being in Anglesey, the Lleyn, Co. Wicklow (near Round- 

 wood), and at Annalong. It may perhaps be one of the " variolites " 

 noted in 1835 by Major Patrickson, who names the rock once or 

 twice in the region to the north of Annalong. 



A very compact black rock, with minute spherical amygdaloidal 

 vesicles, occurs as a thin dyke between Dunmore Head and Green 

 Harbour. It has the high specific gravity of 2-93. In section it 

 approaches in beauty the rhyolitic pitchstones of Arran, while at 

 the same time revealing its more basic character. Feathery bundles 

 and sheaves of felspatbic materials are associated in the pale-green 

 altered glass with long curving rods and plumes, presumably of 

 pyroxene and magnetite. A few porphyritic crystals of plagioclase 

 are again the only constituents sufficiently developed to be identified 

 with absolute confidence. 



1 " Dykes appearing on the shore which skirts the Mourne Mountains." Joiurn. 

 Geol. Soc. Dublin, vol. i. p. 182. 



