Hyland — On some Spherulitic Rocks from Co. Down. 421 



octahedral fluor, topaz, and peridote (Haughton) ; whilst Lacroix 

 has recently added the Yttrium-silicate, Gadolinite, 1 to the list. 

 The presence of drusy cavities, containing white orthoclase and 

 smoky quartz, is another feature which the granites of Mourne 

 and Arran have in common. Intercrystallisation of the quartz 

 and felspars, so as to produce micro-pegmatite, is often to be 

 observed in the Mourne granite. 



The occurrence of the spherulitic rocks now to be described is of 

 interest, and brings the two districts into even closer relationship. 

 Spherulitic " felsites " are, we know, very abundant in Arran, and 

 have received considerable attention at the hands of Allport 2 and 

 Bonney. 3 



I have found spherulitic rocks at three localities in Co. Down, 

 viz. near Newcastle, Hilltown and Slieve Bearnagh. As the most 

 interesting type of this rock occurs at the first locality, I will 

 describe it in full, and content myself with making a short refe- 

 rence to the others at the conclusion of the paper. 



I am indebted to Professor J. P. O'Eeilly, of the Eoyal 

 College of Science, Dublin, for kindly bringing the Newcastle rock 

 under my notice, and supplying me with specimens. It occurs — as 

 a dyke (?) — amongst the Lower Silurian slates, in proximity to the 

 granite of Slieve Donard, near Newcastle, the label reading : "In 

 Lady Annesley's demesne : bed of stream : 200 yds. above bridge 

 near mansion." The direction of the dyke was calculated in 1874 

 to be N. 38° E. The rock is of a grey colour, with greenish hue ; 

 the surface is often marked with ferruginous stainings. With 

 the naked eye the following constituents may be recognised : — 



1 . Felspar : Up to \ inch in length ; colour, pinkish (orthoclase) 



and white (albite) ; at times glassy in appearance ; tabular 

 form. 



2. Quartz : Up to \ inch ; smoky and globular ; pitted with 



minute holes full of a whitish substance. 



3. Chlorite : Mostly as specks. 4 



1 Sur un nouveau gisement de Gadolinite. Bull, de la societe francaise de Mine- 

 ralogie, 1888, xi., p. 68. (See Note added in the Press, p. 436.) 



2 On the Microscopic Structure of the Pitchstones and Felsites of Arran : Geol. 

 Mag. vol. ix., 1872, p. 536 et seq. 



3 On certain Eock-structures, as illustrated by Pitchstones and Felsites in Arran : 

 Geol. Mag. ii., vol. iv., 1877, p. 499. 



4 Amongst the most remarkable dykes which occur traversing both the granite and 



