Vol. 57.]' ALTERED ROCKS FROM NEAR BASTOGNE. 57 



of minute filmy mica and elongated grains of secondary felspar. It 

 is difficult to decide with certainty the nature of the overlying 

 quartz-felspar rock ; on the whole, it seems more probably a 

 crushed grit than a porphyroid, although some of the quartz has 

 the aspect of corrosion by a magma such as is often seen in a quartz- 

 felsite. The rock differs, however, from the porphyroids of the 

 district in not containing the large, much-corroded felspar-crystals. 



The rocks bear a strong resemblance to those from the Llanberis 

 section, North Wales, which include porphyroids and crushed 

 felspathic grits. Near Salm Chateau certain crushed rocks occur, 

 which probably have an origin similar to those of Lamersdorf. 



In other cleaved rocks some of the minerals are certainly secondary, 

 including microlithic white mica, and possibly the better-defined crys- 

 tals of ilmenite, ottrelite, etc. The micromineralogical development 

 is doubtless the result of pressure. For instance, in a gritty phyllite 

 (from St. Pierre) consisting of angular fragments of quartz (with 

 one or two of white mica, biotite, tourmaline, and possibly a 

 zircon), minute crowded mica-films are developed mainly along the 

 cleavage-planes. Of the better-defined secondary minerals, ilmenite- 

 crystals, usually small, occur,^ frequently along cleavage or strain- 

 slip planes, as if their development also might be connected with 

 pressure. The origin of the well-known ottrelite offers some 

 difficulty. It may be partly a result of contact-action, but some 

 indirect evidence rather connects it with pressure-results. It 

 occurs generally in crushed rocks — for instance, one schistose 

 greyish grit from near Yiel Salm, crossed by shining micaceous 

 crush -planes, contains ottrelite-crystals (about 1*5 mm. broad) 

 with rich brown cleavage-faces. The crystals (in this and other 

 rocks) when seen under the microscope have the parallel sides and 

 irregular ends shown by Prof. Renard,^ as if they had grown through 

 the crushed material, like the biotite and hornblende described by 

 Prof. Bonney.^ The last-named author has noticed a mineral re- 

 sembling one of the chloritoid group in a Nufenen rock, where 

 there is neither proof nor any probability of contact-action.* In 

 many slices of the Ardennes rocks patches of ferrite are associated 

 with the ottrelite, as if the isolated crystals may have originated 

 from scattered grains of iron-oxide, which borrowed constituents 

 from the surrounding mass. 



^ See A. Renard, ' Eecherches sur la Composition & la Structure des Phyllades 

 Ardennais' pt. ii, Bull. Mus. Roy. Hist. Nat. Belg. vol. ii (1883) p. 127. 



^ ' Note sur TOttrelite,' A. Renard & Cb. de la Vallee Poussin, Ann. Soe. Greol. 

 Belg. vol. vi (1879) p. 61 & pi. ii, figs. 1-2. Compare the similar form of 

 chloritoid described by Barrois, ' Note sur le Chloritoide du Morbihan ' Bull. 

 Soc. Mineral. France, vol. vii (1884) p. 39. 



^ ' On a Secondary Development of Biotite & of Hornblende in Crystalline 

 Schists from the Binnenthal ' Quart. Journ. Geol. Soc. vol. xlix (1893) p. 104. 



^ 'On the Crystalline Schists & their Relation to the Mesozoic Rocks in the 

 Lepontine Alps ' Quart. Journ. Geol, Soc. vol. xlvi (1890) p. 234. 



