GRANITE OF MULLAGHDERG, CO. DONEGAL. 559 



the point of consolidation of the several minerals has been passed, 

 and, therefore, after their separation is complete, we get, indeed, a 

 concentrically laminated body, but one without a definite arrange- 

 ment of the constituents : this is the well-known spheroidal structure 

 of many eruptive rocks. If, on the other hand, the tendency to 

 form spheroids is developed during the period in which a differentia- 

 tion of the magma into the various minerals can still take place, the 

 latter will naturally undergo a definite arrangement with regard to 

 the central point." 



The first case, as pointed out by Vogelsang, offers a rational ex- 

 planation of that form of spheroidal structure in which there is no 

 definite arrangement of the constituents *. The latter case describes 

 what most probably took place during the formation of spheroids 

 with zonal or radial structure. 



EXPLANATION OF PLATE XIV. 



Fig. ] . A portion of a spheroid, cut through the centre and polished. It 

 shows the light-coloured nucleus, the dark zonal portion, and a small 

 quantity of the granitic matrix attached to the periphery. (Nat. 

 size.) 

 2 1. A portion of the nucleus, viewed in thin section under a low power 

 and between crossed nicols. The striated mineral is oligoclase ; the 

 interstitial colourless substance, quartz ; and the opaque grains, mag- 

 netite. 



3. Section through the zonal portion of the spheroid, seen under a low 



power. The light portions are felspar (oligoclase) ; the black 

 grains, magnetite. The latter have their long axes in the direction 

 of the radii of the spheroid. The dark line in the zonal portion 

 of fig. 1 traverses here the middle of the field, and is seen to be 

 produced by the approximation of the black grains. 



4. Section through the zonal portion of the spheroid, viewed under a low 



power and between crossed nicols. As in fig. 3, the main mass is 

 oligoclase, and the black grains are magnetite. The felspar is seen 

 to be twinned, the twin-lamellse being orientated in the direction of 

 the radii of the spheroid. 



Discussion. 



Mr. EuTLET spoke of the difficulty of doing justice to the immense 

 amount of careful work involved in this paper. The Author had 

 wisely avoided theoretical views, and yet it was tempting to specu- 

 late on the causes which set up crystallization, and to inquire why 

 there was no radial structure in the central portions of the 

 spheroids. 



Prof. Bonnet regarded the paper as a very useful piece of work. 

 He thought difference of structure proved a discontinuity of some 



* In connexion with this point. Prof. Bonney's papers on " Columnar, 

 Fissile, and Spheroidal Structures" (Q. J. G. S. xxxii. p. 140) and "On certain 

 rock structures as illustrated by the pitchstones and felsites of Arran " (Geol. 

 Mag. 1877, iv. p. 429) should be consulted. 



t The microscopic sections are drawn, with the kind assistance of Mr. J. G-. 

 Goodchild, F.G-.S., of the Geological Survey, from photographs taken by Mr. 

 O. Barrow, F.G.S., of the Geological Survey. 



Q. J. G. S. No. 175. 2 p 



