Hyland — On some Spherulitic Bocks from Co. Down. 423 



a sphere polarising in low tints, and showing two dark spots in 

 the centre. Upon further rotation of the nicols these spots en- 

 largen until the black cross becomes once more visible and normal 

 in aspect. Some of the smaller spherules show a modified form 

 of this phenomenon. In their case, the outer portion of a sphere 

 becomes light, the black cross remaining confined to the centre 

 portion. Further rotation of the nicols restores the original 

 aspect of a continuous black cross. 



The larger spherulites immediately attract the attention of the 

 observer by the manner in which they project from the field of vision, 

 and by the brown colour which a finely disseminated " ferritic ' M 

 pigment imparts to them. They are to some extent porphyritic in 

 their occurrence, and represent the globules to be observed on the 

 polished face of the rock. But the large quantity of spherulitic 

 matter in the rock only becomes apparent when a section is studied 

 under crossed nicols. The ground-mass, or matrix, is then seen 

 to consist of innumerable small spherules, cemented together, so 

 to say, by crypto -crystalline (sometimes micro- crystalline) matter 

 in a state of fine subdivision. 



At first sight, these spherulites appear to be identical with 

 those so often observed in glassy rocks ; but a more exact ex- 

 amination reveals that, whereas the latter are homogeneous 

 bodies, those we are now describing are decidedly heterogeneous, 

 and appear to consist partly of felspar, partly of quartz. The 

 felspathic portions would be those which are turbid and ferritic; 

 whilst we would consider as quartz those lines or streaks which 

 are pellucid and free from such brownish substance. The colour- 

 less ring, which surrounds the radial portions, may at times be 

 seen to show traces of micro-pegmatitic structure. Again, the 

 fan-shaped black cross, which most of the spherules present under 

 crossed nicols, is not normal in character — the arms not lying- 

 parallel to the vibration planes of the nicols. These observations 

 prove that we are dealing with those structures which Rosenbusch 

 calls pseudo-spherulites. 2 They probably answer in composition to 



1 "Ferrite" = yellowish, reddish, or brownish amorphous substances or particles, 

 probably consisting of peroxide of iron, either hydrous or anhydrous, but not certainly 

 referable to any mineral. 



2 Mikroskopische Physiographie der massigen Gesteine : Stuttgart, 1887, p. 384. 



SCIEN. PKOC. K.D.S.. — VOL. VI.. PART VIII. 2 K 



