Vol. 57.] ALTERED EOCKS FROM NEAR BASTOGIJE. 67 



with green acieular hornblende often in tufts. The boundary 

 seems clear, but under the microscope it is found to be a slightly 

 brownish zone, containing scattered iron-stained films. Beyond 

 this edge, the hornblende-tufts extend for a few inches within a 

 strong iudurated compact rock, which passes into a pale-grey 

 dusty-looking grit. The patch is probably a section across the 

 crust of a garnetiferous nodule, either still concealed or removed by 

 quarrying. 



Examination with the microscope shows tufts of hornblende, 

 some biotite, iron-oxide, pyrite, and probabl)^ garnet, having a some- 

 what pegmatitic structure. In the grey rock (at about 2 inches 

 from the patch) hornblende-tufts are absent, but biotite, iron-oxide, 

 and small interstitial colourless garnet occur with greenish micro- 

 liths. At about 9 inches are more abundant microliths, and larger 

 and more numerous flakes of biotite, together with ilmenite, along 

 the margin of an included fine-grained band. 



It may be worthy of observation that the patches of altered rock 

 are either black, containing carbon ^ and garnets accompanied by 

 ottrelite or hornblende ; or they are greyer, more compact, crowded 

 with tufts of hornblende without the sharp-edged garnets. Both 

 rocks are fine-grained grits, but the latter is rather coarser, with a 

 texture approaching that of a quartzite." 



In each of the cuttings described the strata form a very flat, 

 slightly undulating anticline, each cusp which points downward 

 corresponding with a vertical joint, filled by secondary quartz, 

 thinning out below. 



III. Theoretical Considerations. 



As to the causes which have affected these rocks, Prof. Gosselet 

 maintains that the metamorphism is explained almost entirely by 

 mechanical action, except in the sahlbands of the porphyroids and 

 of the granite. The formation of new minerals was due, according 

 to him, to the heat produced by arrest of movement, compression, 

 and friction. The movements continued over a long period. Some 

 rocks enclosing crystals produced by an earlier metamorphism 

 were laminated by later pressure (' phyllades aimantiferes ' of 

 Deville). In others which were already ' phyllades,' crystals 

 were developed b}' a second metamorphism (as, for example, the 

 ottrelite-schist of Bogny). The chapter which advocates this view ^ 

 is a most valuable statement of facts and observations ; but, for 

 reasons of which the following is a summary, I am not convinced 

 by the arguments : — 



The author maintains that the ' corneite ' is due to a more intense 

 pressure, where the rocks are folded over to the north, yet he 



^ See Bull. Mus. Roy. Hist. Nat. Belg. vol. i (1882) pp. 15, 17. The graphite, 

 as described by Prof, Renard, is often disseminated as a fine dust. 



- Compare the classification by Prof. Renard into quartzites, grits, and 

 ' phyllades ; ' oj). cit. p. 7. 

 . 3 ' L'Ardeniie,' 1888, eh. xxt. p. 759. 



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