Cott & CunnrincHam—On Certain Rocks styled “‘Felstones.”? 319 
of consolidation. In a specimen in which the felspar microlites 
are better developed in the groundmass, the species proves, by its 
optical characters, to be andesine. 
The zoned and twinned porphyritic amphibole reminds one of 
that in the camptonite of Roda in Tyrol,! and the colour is some- 
what similar. The ground of the Roda rock is, however, more 
completely differentiated, and is crowded with small brown idio- 
morphic crystals of amphibole. As we have already stated, the 
amphibole in the rock described by Dr. Hyland from Aughagault 
seems to agree with ours in all respects; and, in the decomposed 
parts of the Convoy dyke, the alteration-products of the larger 
groups of brown amphibole go far towards reproducing the 
complex pseudomorphic patches which are a feature of the 
camptonite of Aughagault. 
The grouping of the amphibole crystals in the Convoy dyke 
here and there suggests that they may have been derived from the 
absorption of inclusions of foreign material. The arrangement 
appears, however, to be truly “ glomeroporphyritic,”’ and detached 
erystals have often been corroded by the groundmass, which has 
finally crystallised within them. The groundmass has similarly 
intruded itself between the components of the groups, and is 
easily recognised by its characters between crossed nicols. At the 
same time, a colourless substance, with very striking phenomena 
of absorption when tested with a single nicol, is seen to unite 
many of the amphibole crystals in the groups. Microchemical 
examination proves this to be calcite; but its mode of occurrence 
makes it clear that it occupies the place of some pre-existing 
mineral in the groups. Possibly it represents a felspar ; possibly 
it results from the alteration of a felspathoid. In any case, it 
offers a puzzle similar to those presented by the colourless con- 
stituent of the monchiquites, a family of rocks now so much 
under discussion. A certain amount of anisotropic material, with 
weak double refraction, remains in the interspaces between the 
hornblende after the calcite has been dissolved away. (Pl. XIX., 
fig. 2.) 
Apatite occurs in the groundmass of the dyke, and is also 
included in the porphyritic amphibole. Though lighter in colour, 
1 This rock is described by Rosenbusch, ‘‘ Mikroskop. Physiographie der mass 
Gesteine,’’ 3te Auflage (1896), p. 546. 
SCIENT. PROC., R.D.S., VOL. IX., PART III. 2A 
