558 DR. F. H. HATCH ON THE SPHEROID-BEARING 



stituted granite, or of one or more crystals of orthoclase ; or, again, 

 of an aggregate of plagioclase grains. Sometimes it is an accumu- 

 lation of granules of magnetite * ; sometimes one of the dark mica- 

 ceous nodules of group It. As the concretion forms round the 

 original nodule, its irregularities are gradually equalized; so that 

 the spheroidal or ellipsoidal character is soon acquired. 



With regard to the genesis of the spheroids, there can be little 

 doubt that they have been formed by concretion during the con- 

 solidation of the granite-magma ; they must, consequently, be re- 

 garded as an example of zonal and, in the case of the belonosphe- 

 rites, radial crystallization around an earlier-formed nucleus. As 

 one of the conditions for the development of such a crystallization a 

 quiescent condition seems a -priori to be essential. Differential 

 movements in the granite-magma, which have resulted in a consider- 

 able distortion of the spheroids, must in some cases, however, have 

 taken place before the final consolidation of the rock %. We have 

 here an instance of fluxion-structure, which, in granite, is undoubt- 

 edly a rare phenomenon. Chemical composition appears also to be 

 a determining factor ; for in at least two of the above classes the 

 concretions are more basic than the enclosing granite, the intervening 

 material, on the other hand, being of normal constitution. A defi- 

 ciency in silica would thus seem to favour the development of spheroids 

 in granite. Whether the occurrence of basic patches be due to the 

 absorption of pre-existing basic material or to some process of segre- 

 gation, must remain, for the present, an open question. Brogger §, 

 on the other hand, hazards the conjecture that the spheroidal structure 

 is preferably developed along the marginal portion of the granite 

 area, i. e. in those portions where the magma fii*st consolidates. This, 

 he says, would be analogous to the occurrence of granophyric or 

 porphyritic structure in the marginal portions or in the apophyses 

 of granite or syenite. The balance of evidence however, scarcely 

 seems to favour this hypothesis. It certainly will not explain, for 

 example, the occurrence at Mullaghderg, Co. Donegal ; for this locality 

 lies right in the midst of the granite-area ||. 



With reference to the formation of these remarkable bodies, 

 perhaps it would be not unprofitable to recall to mind Vogelsang's 

 explanation of similar phenomena, given in his admirable paper 

 describing the concretionary balls of the Kiigeldiorit (^ISTapoleonite) 

 and Kugelporjphyr of Corsica ^ : — "When a molten magma consoli- 

 dates, an irregular {unghiclimdssig) cooling may produce greater 

 contraction of the mass at certain points ; and this may lead, later 

 on, to a spheroidal separation. If this condition is arrived at after 



* Zirkel, I. c- t Vom Rath, Z. c. 



X Brogger, I. c. pp. 323, 327. § Loc. cit. p, 344. 



II Mr. Kilroe writes : — " The mass of spheroidal concretions does not occur 

 at or near the margin of the granitic area. The point is almost equidistant 

 from quartzite, occurring in mass some miles away on either side, through 

 which the granite has been intruded. Small isolated masses of quartzite and 

 schist are included in the granite in various places, but none near the special 

 point referred to." 



% Sitzungsber. d. uiederrhein. Ges. 1862, xix. p. 185. 



