Vol. 57.] ALTERED fiOCKS FEOM NEAR BASTOGNE. 71 



regular shape, and regularly-arranged inclusions ^ point to some 

 special action. Doubtless favoured by the slow growth of well- 

 formed crystals, inclusions accumulated along crystallographic planes, 

 and material from the groundmass was pushed to the exterior, so 

 that a border of carbonaceous dust, or sometimes of secondary 

 crystalline mineral, is found. Larger garnets or groups of them are 

 sometimes developed along joints or planes of bedding. Either the 

 infiltrating waters may have permeated more readily here, or 

 crystallization may have started from the divisional surface. 



The tufted growth of acicular or platy minerals (such as actinolite) 

 has been shown in other rocks to be due to crystallization overcoming 

 the resistance of a crushed or a fine-grained powdery mass.^ But 

 here we claim an initial aid to crystallization in the presence of 

 heated waters. Even the friable earthy character of the interior 

 might be caused by the later effects of vapours and solutions decom- 

 posing the mass. 



Further, the numerous joints and the quartz-veins, narrowed 

 downward towards the altered rock, might be due to chemical 

 action. The expansion of the rock caused in such changes would 

 slightly lift the overlying strata, forming a low arch, with joints as 

 described above. 



The solfataric theory may claim, to a certain extent, the support 

 of Prof. Gosselet, since he emphasizes the important part which he 

 believes that superheated water has played ^ ; but I cannot adopt his 

 view that the heat was developed through mechanical disturbances. 



Thus we consider that this district exhibits modifications due to 

 different kinds of action. Compression and folding, probably at more 

 than one epoch, produced slaty cleavage, schistose structure (in- 

 cluding that of the peculiar squeezed porphyroids), and even initiated 

 the development of certain minerals. Contact-metamorphism, due 

 to subjacent masses (like the granite which at Lamersdorf rises 

 actually to the surface), probably acted over a certain area ; while 

 the local action of hot springs induced the development of the 

 peculiar garnetiferous and hornblendic rocks. 



Discussion. 



Gen. McMahon remarked that the beautiful lantern-illustratious 

 shown on the screen seemed to him to be quite typical examples of 

 contact-metamorphism acting on fine-grained sedimentary rocks. 



^ We may compare the sharp form of couseranite within a shghtlj-ebanged 

 ground ; also the sharp-edged pyreueite (in a blackish limestone) which has 

 similar regularly-placed inclusions. This latter is described by M. Ed. Mallard 

 as corresponding to a group of orthorhombic pyramids, Bull. Sec. Mineral. 

 Franc, vol. xiv (1891) p. 293. 



2 See T. G. Bonney, Quart. Journ. Geol. Soc. vol. Hv (1898) p. 371 ; Proc. 

 Eoy. Soc. vol. Ixiii (1898) p. 220; T. G. Bonney & C. A. Raisin, • On Varieties 

 of Serpentine & Associated Rocks in Anglesey ' Quart. Journ. Geol, Soc. vol. Iv 

 (1899) pp. 294-97. 



3 ' L'Ardenne ' 1888, p. 762. 



