GRANITE OF MULLAGHDERG, CO. DONEGAL. 555 



broad dark-coloured peripheral shell. They are evidently more 

 basic than the surrounding granite ; while of the spheroids them- 

 selves the marginal portion is more basic than the nucleus, the 

 latter consisting principally of triclinic felspar together with a little 

 quartz and possibly orthoclase, the former of oligoclase with abun- 

 dant included plates of biotite and over 12 7o of magnetic iron-ore. 

 The felspar of the zonal portion is disposed radially, the iron-ore 

 radially and concentrically, while no fixed law appears to govern 

 the distribution of the mica. The concentric accumulation of the 

 magnetite may be compared to the zonal inclusion of foreign minerals 

 in leucite or in the felspars of andesitic rocks. 



Literature. — Concretionary bodies have long been known to occur 

 in granite ; but these, though often of spheroidal shape, have gene- 

 rally been described as destitute of radial structure. A concentrically 

 laminated structure, frequently only rendered visible by weathering, 

 is, however, not uncommon. 



The first mention I have been able to find of such bodies is in 

 Leopold V. Buch's ' Geognostische Beobachtungen auf E-eisen durch 

 Beutschland und Italien'*. On p. 16 he describes, as occurring in 

 an intrusive granite of the Riesengebirge (" Felsen des Kynasts " 

 near Warmbrunn, Silesia), balls of a very fine-grained granitic 

 material, which towards the exterior are richer in mica than in the 

 central portion. Gustav Hosef, in 1842, gave a more detailed de- 

 scription of these bodies. His specimens were derived from Schwarz- 

 bach and the Kynast. He describes balls of 6 inches diameter. 

 The central portion of these is formed by one or more twinned crystals 

 of orthoclase, around which lies a narrow zone of albite and mica. 



C irious concretionary spheroids in the granite of Chanteloube 

 (l)epartement de la Haute-Yienne) are described by Alluaud J. 

 The dimensions of these bodies are from 50 cm. to 1-2 metres. 

 They consist essentially of a nucleus of orthoclase, surrounded by a 

 finely granular aggregate of quartz and felspar, the quartz gradually 

 diminishing in quantity towards the periphery. 



Charpentier §, Collomb I|, Jokely^, and von Andrian** mention 

 similar bodies. 



Zirkel tt describes, in the granite of Clark's Peak, Medicine Bow 

 Range, U.S., small aggregates of granular magnetite surrounded by a 

 fine-grained zone of quartz, felspar, and muscovite. 



Not unfrequently the concretions consist, for the most part, of 



* Vol. i. (1802). 



t Poggend. Ann. toI. Ivi. 1842, p. 624 ; and J. Roth, Erlauterungen zu 

 der geognost. Karte vom niederschles. Geb. &c. 1867, p. 63. 



+ Bull de la Soc. geol. de Fr. (2), vii. 1850, p. 230. 



§ 'Essai sur la constitution geognostique des Pyrenees,' 1823, p. 132. 



II Bull, de la Soc. geol. de France (2), vii. 1850, p. 297. 



^ " Geognost. Verhaltnisse in eineui Theile des mittleren Bohmens," Jalirb. 

 k.-k. geol. Reichsanst, 1855, p. 375. ^, 



** " Beitrage zur Geologie des Kaurimer und Taborer Kreises in Bohmen," 

 Jahrb. k.-k. geol. Reichsanst. 1863, p. 166. 



tt Microscopical Petrography of the 40th Parallel, 1876, p. 53. 



