Yol. 57.] 



ALTERED EOCKS FROM NEAR BASTOGI^E. 



63 



grained, grey, rather dusty, faintly laminated, with no evident 

 distinctions. 



The rock directly adjoining the nodule, when examined under the 

 microscope, is seen to consist mainly of clear quartz, and altered or 

 secondary felspar, with abundant greenish microliths ; biotite, 

 chlorite, (?) epidote, and some zircons occur, all small. The external 

 zone of the nodule has a clearer groundmass without the microliths 

 or small flakes, though with small (?) epidote, but secondary 

 minerals of more conspicuous size now appear. Sheaves and tufts 

 are formed mainly of a green actinolite,^ while certain highly 

 refractive colourless granules often aggregated are probably epidote. 

 The next browner zone seems to owe its colour to hgematiie, which 

 forms either minute patches (sometimes within the hornblende) or 

 a surface- deposit on grains of the groundmass. The colour of the 

 centre is caused by the preponderance of black dust, probably carbon, 

 which sometimes saturates the felspar as if formed from a carbon- 

 aceous mud. Thus successive zones are characterized by : — 

 (1) clearer crystalline grains of the groundmass with microliths ; 

 (ii) still clearer groundmass ; crystals of hornblende, some granu- 

 lar epidote ; (iii) in addition, deposit of iron-oxide (haematite, etc.) ; 

 (iv) in addition, deposit of carbon. 



Fig. 2. — Banded rock including a garnetiferous nodule ; nortJi-east 

 of Bastogne, hy the road, to Longwilly. 



[The nodule measures about 2 feet across ; is black, with a greyish margin ; 

 and contains small reddish garnets and small * ottrelite.' The laminae 

 above the nodule are interrupted and slightly disturbed, along cracks filled 

 by quartz-veins, narrowing downward.] 



The projecting band previously mentioned is more compact and 

 less carbonaceous, and contains actinolite-tufts, some of which grow 

 into it from the adjacent rock. In the thinner |-inch band, the tufts 

 are even clearer, projecting inwards or curving along the surface. 

 They apparently grow more readily through fine-grained material,^ 



1 See Bull. Mus. Koy. Hist. Nat. Belg. vol. i (1882) pi. i, fig. 2, pi. ii, fig. 1, & 

 pi. iii, fig. 1. 



■^ T. G-. Bonney, 'The G-arnet-Actinolite Schists on the Southern Side of 

 the St. Gothard Pass' Quart. Journ. Geol. Soc. vol. hv (1898) p. 371. See 

 also Proc. Eoy. Soc. vol. Ixiii (1898) p. 220. 



