422 Scientific Proceedings, Royal Dublin Society. 



Upon the polished face of the rock the greenish-grey matrix 

 is observed to be studded with small globules of a whitish mate- 

 rial. The quartzes are also seen to be surrounded by a film of the 

 same substance. Under the microscope these globules are seen to 

 be spherulites. A crystal of quartz often forms a nucleus ; at other 

 times no nucleus is apparent. The porphyritic felspars are frequently 

 encircled by tufts of this radiating crystalline matter. The spheru- 

 lites consist of alternating brown and colourless streaks of crystal- 

 line material ; a radial structure is apparent and is usually made 

 apparent by disseminated brown, dust-like matter, which is ar- 

 ranged along certain lines. The radiation from a common centre 

 is not always uniform, and a sheaf or bundle of " fibres " often 

 stands across others which possess one and the same centre. The 

 form is not always that of a well-defined circle. Further, it often 

 happens that the spherulites have been developed side by side, in 

 which case the spherical form of the individuals is more or less de- 

 stroyed by mutual interference. Still, a polygonal network does not 

 often result from this interference. The radiating portion of the 

 spherulites is surrounded by an almost colourless ring, the par- 

 ticles of which possess the same optical orientation as that of those 

 composing the inner portion. Hence, under crossed nicols, these 

 bodies show a four-armed black cross, which is continuous through 

 the outer ring. Following upon this colourless ring, a further 

 development of radiating matter is sometimes to be observed. 

 The black cross is fan-shaped towards the circumference of these 

 bodies, and the arms may not lie parallel to the vibration planes 

 of the nicols. The cross is not infrequently disturbed upon 

 rotation of the nicols. It may either " open " to a small extent, 

 or it may almost entirely disappear. In the latter case we obtain 



slate, Lieutenant James describes " one visible in tbe large crater-sbaped hollow above 

 Tullamore Park ; on the western side of Corra it is about 50 feet thick, and is com- 

 posed of cream-coloured porphyry, containing globular smoked quartz, with crystals 

 of felspar." This is possibly a similar rock to that which I am now describing. (On 

 the Mourne Mountains: Paper read before Geol. Soc, Dublin, Jan. 14, 1835 ; see also 

 Presidential Address by Griffith, Feb. 11, 1835, pp. 21-2S.) G. A. J. Cole has 

 mentioned tbe occurrence, and studied the characters, of a spherulitic tachylyte which 

 is found as the selvage of a basalt dyke at an elevated point of the high-road from 

 Newcastle, Co. Down, west of Bryansford, and near tbe afore-mentioned Tullamore 

 Park. (On some Additional Occurrences of Tachylyte, Q. J. G. S., Lon., May, 18SS, 

 xliv., p. 305, and PL xi., fig. 4.) 



