60 MISS C. A. RAISIN OlS CEETAIN [Feb. I9OI, 



(3) The Garnetiferous and Hornblendic Rocks. 



•Previous observers bave called attention to the very limited 

 distribution of these rocks in the field. Dumont noticed many 

 examples/ vrhich, as later authorities state, cannot now be seen. 

 Prof, Gosselet quotes Dumont, and figures some drawings showing 

 their mode of occurrence.^ They are sometimes in rather short, 

 generally lenticular bands, sometimes in ' nodules.' ^ Thus they are 

 quickly worked out in the quarries, and loose fragments of them 

 are few and scattered, preserved doubtless only by their superior 

 hardness. 



The petrographical characters of hand-specimens have been 

 exhaustively described by Prof. Renard. The minerals, which he 

 enumerates ^ as seen under the microscope, in the garnetiferous and 

 hornblendic rocks and the ' phyllades,'* are : — 



(1) Graphite as fine dust, or as hexagonal scales ; (2) iron-oxide often 

 plate-like (haematite or ilmenite) ; occasionally magnetite is present ; (3) titanite ; 

 (4) zircon ; (5) apatite ; (6) quartz ; (7) white mica. 



In the ' phyllades ' : (8) rutile, (9) tourmaline, (10) ottrelite (with sillimanite, 

 pyrite, pyrrhotine), (11) and biotite, similar to that which I have described in 

 the previous section, p. 59. 



(12) Garnet often in sharp well-developed dodecahedra, with regularly 

 arranged enclosures. I have also found garnet interrupted by much of the 

 groundmass in a manner somewhat resembling micropegmatite. 



(13) Hornblende in sheaves or tufts. 



To these I would add the following minerals, some of which I 

 have identified with hesitation in the absence of certain distinctive 

 characteristics ; and where the minerals are mentioned by name in 

 succeeding pages, this must be taken only as the most probable 

 identification : — 



(14) Felspar (described by Prof. Gosselet). 



(15) A mineral which forms crystals with straight sides, ragged ends, and 

 a parallel cleavage, resembling mica or more closely an ottrelite : the crystals 

 sometimes develop incurving tufts like the hornblende. The mineral always 

 includes grains of the groundmass, which are sometimes so abundant that the 

 crystals are scarcely more than suggested. Except for an occasional yellowish 

 tint, probably iron-staining, it is practically colourless, but, on close examina- 

 tion with a high power, sho\vs a very slight green. It is not at all, or very 

 faintly pleochroic ; and with crossed nicols its polarization-tints are so low as 

 often to be barely perceptible — then dull greenish or yellowish-green. It has 

 polysynthetic twinning like plagioclase, the above-named colours being striped 

 with dull grey, green, cr leaden blue. The mineral has at least one very well- 

 marked cleavage, sometimes mica-like but more usually separating lath-shaped 

 films ; besides cross-cleavages which make with this an angle difficult to 

 determine, because of the curvature. The twinning-stripes are occasionally 

 transverse to the perfect cleavage. The mineral includes granular opacite, 

 sometimes scattered irregularly, but generally aggregated along the cleavage- 



^ ' Memoire sur les Terrains Ardeunais & Rhenan ' p. 305. 



- ' L'Ardenne ' (1888) pp. 787-91. 



3 Ibid. p. 785. Prof. Gosselet says that the ' metamorphic rock ' occurs gene- 

 rally as ' nodules.' Prof Renard says that it occurs .... 'en lits ' .... 'en 

 bancs minces '....' sous la forrae d'amas couches ' .... ' sous la forme de 

 nodules:' see Bull. Mus. Roy. Hist. Nat. Belg. vol. i (1882) p. 7. 



^ ' Les Roches Grenatiferes & Amphiboliques de la Region de Bastogne ' Bull. 

 Mus. Roy. Hist. Nat. Belg. vol. i (1882) pp. 1-47; 'Recherohes sur la Compo- 

 sition & la Structure des Phyllades Ardennais ' Ibid. p. 215 & vol. ii (1883) p. 127. 



