^0^' 57*] ALTERED ROCKS FROM: NE.VR BA.STOGNE. 69 



altered examples, the biotifce is generally more abundant, but the 

 greatest amount is in one specimen from Libramont, and in all it 

 is similar in character. Again, the small microscopic garnets in 

 rocks from Libramont are rounded or subrotund, rather like those 

 in contact-specimens from Glendalough and from ISTew Galloway, 

 although in the former the garnets have a grey centre and are 

 contained in a fairly coarse mica-schist/ 



Thus a resemblance is exhibited to contact-met amorphic rocks 

 from other localities. As to the cause, it is true that, as Prof. Eenard 

 says,^ the veins of porphyry and diorite in the Ardennes do not occur 

 where the metamorphism is intense, but such intrusions as these 

 would not cause much change. If the metamorphism in Brabant 

 is due to them (as claimed by Dumont), it probably is only where 

 the change is slight. Further, the existence of a subjacent igneous 

 mass is surely not a ' gratuitous hypothesis,' ^ when evidence of its 

 results can be given as stated above, and when in one district a 

 granite has been shown to be exposed. 



At the same time, the development of garnet and hornblende is 

 so local and limited, that we seem forced to attribute them to a 

 somewhat different cause. In regard to these, the following facts 

 may be established : — 



The stratijfi cation passing almost horizontally above the top of 

 the ^ nodule ' belongs to a low, somewhat undulating anticline 

 (although compression has given rise to slaty cleavage in some 

 layers at a short distance). Thus the metamorphism cannot be due 

 to folding or mechanical disturbance of the rocks, and the ' nodule ' 

 could not be a curiously contorted part of a band. Though the 

 line of demarcation seems rather sharp in the field, and suggestive 

 of a junction of an igneous and metamorphic rock, microscopic 

 examination shows a gradual passage, with continuous lamination. 

 The ' nodule ' is part of the surrounding rock metamorphosed. 



The ' nodule ' is always very limited, generally a few feet across, 

 and surrounded by normal, or but slightly altered, sedimentary rock. 

 It is not likely to be a projecting knob of an ordinary contact-zone 

 around a subjacent igneous mass, for such is nowhere exposed,^ 

 and its position within the surrounding rock makes this almost 

 impossible. 



Further, the secondary minerals differ from those of an ordinary 

 contact-zone in certain respects, although resembling them in others. 

 We note, for example: (1) the absence of any large andalusite ; 

 (2) the tufted growth of the hornblende ; (3) the sharp outline of 

 the garnets in a comparatively unmetamorphosed groundmass ; 

 (4) their peculiar internal structure ; and (5) the frequent presence 



^ The garnets of the Brazil-Wood gneiss ha^e a sharper outline, and exhibit 

 internal cracks ; but the rock is of a different type. 



2 See Bull. Mue. Roy. Hist. Nat. Belg. vol. i (18S2) pp. 35, 36. 



3 'L'Ardenne' 1888, p. 761. 



* By the road from Bastogne to Longwillj- no igneous rock is seen in the 

 lowest part of the quarry, as already stated ; and I searched the lower cutting 

 by the railway, and the craglets south of the valley, but found none. 



