552 DR. F. H. HATCH ON THE SPHEEOID-BEARIXG 



place oligoclase high up in the order of consolidation of minerals in 

 granite. 



The matrix intervening between the balls differs but little from 

 the granite just described. Its texture is a little finer, and it con- 

 tains perhaps a somewhat greater proportion of oligoclase. 



The Spheroids. — The most perfect of the concretionary bodies 

 submitted to me for examination is a somewhat flattened spheroid, 

 the greatest diameter of which is about four inches, and the smallest 

 three. A thin layer of reddish felspar, with which abundant 

 glistening plates of a dark-coloured mica are associated, is the only 

 remnant of the enveloping matrix remaining attached to the peri- 

 pheral surface. This felspar is orthoclase of specific gravity 2-571, 

 which, it will be seen, is almost identical with that of the orthoclase 

 in the surrounding granite. The mica, too, presents optical pro- 

 perties similar to those of the mineral described above. 



In order to investigate its internal structure, the spheroid was 

 cut through the centre, and one of the halves polished. Sections * 

 showing both central and marginal portions of this and other 

 spheroids were also prepared. 



Two distinct parts were then clearly to be made out : — a pink 

 felspathic nucleus, and a dark-coloured peripheral shell. Measured 

 in section, the diameter of the nucleus is about 1^ inch; while 

 the width of the external zonal portion is about 1 inch. The outer 

 periphery of the spheroid is regular and sharply defined, being 

 marked oft' from the granitic matrix in which it is imbedded by the 

 dark colour of the marginal portion. A closer examination discloses 

 the fact that this dark colour is not, as at first sight appears, uniformly 

 diff'used ; for the main mass is found to consist of a colourless and 

 transparent mineral, enclosing innumerable granules of a black, 

 opaque substance, with metallic lustre. Nor are the granules with- 

 out order in their distribution, being disposed in more or less definite 

 rings so as to produce a marked concentric structure (see fig. 1, 

 PL XIV.). 



The boundary of the nucleus, on the other hand, is irregular and 

 wanting in sharpness. This is caused by an almost imperceptible 

 transition (due to a gradual increase in the number of black granules) 

 of the nucleus into the outer zonal portion. 



Submitted to microscopic examination, the nucleus is found to 

 consist of an irregular hypidiomorphic granular aggregate, chiefly 

 of tri clinic felspar, presenting well-defined twin-striation often on 

 both albite- and pericline-types. This twinning is extremely sporadic 

 in its occurrence, one and the same grain often showing no trace of 

 striation in one place, while in another the twinning is well marked. 

 Though occasionally clear and fresh, the felspar has oft<en sufi'ered 

 from kaolinization, turbid patches being developed in consequence- 

 Such alteration generally follows the cleavage-cracks, but in some 



* I have great pleasure in expressing my admiration of the waj' in which 

 these sections have been prepared by Mr. J. Young in the laboratory of the 

 G-eological Survey at Edinburgh. Though of unusually large size, they are of 

 sufficient thinness to show the minutest structural details. 



