Hyland — On some Spherulitic Rocks from Co. Down. 429 



Silurian rocks at Leitrim River, Hilltowu 1 (Sheet 60 (1 inch) of 

 Geol. Survey of Ireland.) Upon examination of this rock, under 

 the microscope, I find it to be spherulitic, and almost identical in 

 character with the rock from near Newcastle. The spherules 

 are here more apparent in ordinary light, owing to the excessive 

 amount of ferruginous dust with which they are besprinkled. 

 The contrast between the radiating nucleus and the only slightly 

 coloured peripheral portion is thus very marked. The globular char- 

 acter of the spherules is very decided at times. Orthoclase is often 

 present as Carlsbad twins ; but little plagioclase is apparent. The 

 quartz frequently shows excellent crystallographic boundaries, the 

 prism being not an uncommon face to be observed. Secondary 

 quartz is found encircling many of the spherules, and is constantly 

 observed throughout the matrix of the rock. A dark mica 

 (biotite) is also present in small quantity. Its pleochroism is 

 between golden-yellow and dark-brown. It is also found as thin 

 threads, scattered sporadically through the groundmass. This 

 latter is, at places, decidedly micro-crystalline. 



The third locality for rocks of this description is Slieve Bear- 

 nagh. The rock is the central dyke, traversing the Mourne 

 granite at the summit of the mountain in an E.S.E. dii-ectiom 

 "It is of a light-brown or gray colour, consisting of a felsitio 

 paste enclosing crystals of felspar (often decomposed) and speckled 

 by a greenish mineral." 2 This rock is also spherulitic, but not 

 to the same degree as that from Newcastle. Both orthoclase and 

 plagioclase are present. Carlsbad twinning is frequent. The 

 plagioclase shows both the albitic and periclinic striae. This 

 rock presents an excellent example of growth of crystalline mate- 

 rial after consolidation. 3 For instance, a felspar crystal which 



1 The " reddish granitoid dyke, projecting S. from the Newry Granite at Hilltown," 

 is seen upon microscopic examination to he a granophyre (Rosenbusch) : the ground- 

 mass is micropegmatitic. The felspar of the micropegmatite surrounding a porphyritic 

 felspar crystal mostly extinguishes the light simultaneously with the crystal, and where 

 the latter is a Carlsbad twin, the trace of the twinning plane may be followed into the 

 micropegmatite. The felspar crystals are generally unstriated, and Carlsbad twins are 

 very frequent. A greenish or bluish-green micaceous mineral is present, also a white 

 radiating mica (muscovite). 



3 Prof. Hull, in Memoir to Sheets 60, 61, &c., 1881, p. 36. 



3 Cf. Judd, in Q. J. G. S., Lon., vol. xlii. (1886) pp. 72, 73, pi. vii. fig. 8 ; also 

 ibid, for May 1889, p. 175 et scqq. ; further, E. Haworth, The Archaean Geology of 

 Missouri : Minneapolis, Minn., 1888, pp. 16, 17, and Amer. Geologist, May and June, 1888. 



