GRANITE OF MrLLAGHDERG, CO. DONEGAL. 557 



An interesting variety of spheroid has lately been discovered in 

 the Stockholm granite of the Scandinavian peninsula, and has 

 been the subject of a searching examination and detailed description 

 by W. C. Brogger and H. Backstrom *. The spheroids occur in 

 several distinct masses in a granitite consisting of orthoclase (in part 

 microcliue), oligoclase, quartz, and dark mica. Each ball is made up 

 of a dark nucleus and an outer light-coloured zone. The former is 

 composed of the same minerals as the surrounding rock, the latter 

 principally of felspar (plagioclase, orthoclase, and microcline) to- 

 gether with quartz. Dark minerals (mica, &c.) are entirely absent 

 from this portion of the spheroid. The texture of the outer zone is 

 granular, but finer-grained than the nucleus or the surrounding 

 granite. A radially divergent structure was observed by Brogger 

 in one instance only. The nucleus passes by imperceptible grada- 

 tions into the peripheral portion ; the latter, however, is sharply 

 separated from the granite. No fixed proportion exists between 

 inner kernel and outer zone. In some few cases the granitic material 

 of the nucleus was found to be replaced by a single large individual 

 of felspar (microcline), as in the spheroids described by G. Eose (cited 

 above). 



Conclusions. — The concretionary bodies, briefly described in the 

 above resume, may be classed as follows : — 



I. Local accumulations of dark mica, hornblende, and a little 

 triclinic felspar, generally of irregular shape, but sometimes rudely 

 spheroidal. These are the " concretionary patches " of Phillips. 



II. Spheroids composed of quartz, felspar, and mica or hornblende 

 (?'. e. the same minerals that compose the surrounding rock) in gra- 

 nular aggregation. These bodies possess an inner kernel and an outer 

 zone, the latter sometimes exhibiting concentric structure. Brogger f 

 subdivides them into (1) those having a nucleus relatively rich in 

 dark minerals (biotite, hornblende, &c.) with a peripheral portion 

 relatively poor in, or free from, dark minerals ; (2) those with a 

 peripheral zone rich in dark minerals. The whole group may per- 

 haps be classed with Yogelsang's granospherites t. 



III. Spheroidal aggregations of triclinic felspar with dark mica, 

 hornblende, or magnetite around a central nucleus. This variety is 

 characterized by the possession of both radial and concentric structure, 

 and may be referred to the helonospherites of Yogelsang§. To this 

 group belong the spheroids from Mullaghderg, as also those from 

 Tonui. 



The nucleus of the spheroids is extremely variable, both as regards 

 size and composition. It may consist of a portion of normally con- 



* " Om forekomsten af 'klotgranit' i Vasastaden, Stockholm," Greol. Foren. 

 Stockh. Forhandl. no. HO, Bd. ix. Haft. 5, p. 307. 



I am indebted to Prof. Tornebohm for directing my attention to this impor- 

 tant paper, as also for kindly supplying me with specimens, for the purpose of 

 comparison, from both the Swedish localities (Slatmossa and Vasastaden). 



t Loc. cit. p. 351. 



X ' Die Krystalliten,' Bonn, 1885, p. 134. 



§ Loc. cit. 



