^Ol' 57'] ALTEEED EOCKS NEAR BASTOGNE. 55 



5. On CEETATN Alteeed Rocks from near Bastogne,^ and thete 

 Eelations to OTHEEs in the Disteict. By Catheeine A. Baisin, 

 D.Sc. (Communicated by Prof. T. G. Bonnet, D.Sc, LL.D., 

 P.B.S., P.G.S. Bead November 7th, 1900.) 



Contents. 



Page 



I. Introduction 55 



II, Mineral Modifications 56 



(1) Results of Pressure. 



(2) Probable Contact- Alterations. 



(3) The Garnetiferous and Hornblendic Rocks. 



III. Theoretical Considerations 67 



I. Inteoduction. • 



Much has been written about the exceptional rocks of the Ardennes, 

 but the petrographical work has been treated, in some memoirs, 

 apart from the field-evidence. This, indeed, owing to the limited 

 occurrence of the most peculiar specimens, is not easy to obtain. 

 It seemed then that a detailed study of a few interesting examples 

 of the rocks, and of their relations in the field, might be worth a 

 brief record. Further, eminent authorities have expressed different 

 opinions as to the cause of the alterations. Andre Dumont in his 

 famous monograph^ described numerous observations, and inclined 

 to the view that the structures were the result of contact-action. 

 Prof. Barrels,^ from comparison of specimens with those of Britanny, 

 supported this theory. A. von Lasaulx * added the important evidence 

 that a granite occurs in the Hohe Yenn. Other authorities, how- 

 ever, have described the folding, contortion, and faulting of the 

 rocks, and attribute the changes to mechanical influences. This 

 opinion is advocated by Prof. Benard in a valuable petrographical 

 paper,' and by Prof. Gosselet in his exhaustive memoir on the dis- 

 trict.^ Thus I became interested in the question, and, at the desire 

 of Prof. Bonney (as he was unable at that time to undertake the 



^ [Bastogne is a small town in the extreme north-east of the Belgian 

 Ardennes, a few miles away from the frontier of the Grand Duchy of Luxem- 

 burg.] 



^ ' Memoire sur les Terrains Ardennais & Ehenan ' p. 232. [See also Mem. 

 Acad. Roy. Belg. vol. xx, 1847.] 



^ Ann. Soc. Geol. Nord vol. x (1883) p. 205. The comparison here is drawn, 

 however, with the porphyroids. 



'^ ' Der Granit unter dem Cambrium des Hohen Yenn' Yerhandl. Naturh. 

 Ver. Preuss. Rheinl. vol. xli (1884) p. 418. See also E. Dupont, Bull. Acad. 

 Roy. Belg. ser. 3, vol. ix (1885) pp. 110-114. 



' A. Renard, ' Les Roches Grenatiferes & Amphiboliques de la Region de 

 Bastogne ' Bull. Mus. Roy. Hist. Nat. Belg. vol. i (1882) pp. 33-39. 



6 ' L'Ardenne ' (1888) p. 762. See also Eugen Geinitz, ' Der Phyllit von 

 Rimognes in den Ardennen ' Tscherm. Min. & Petr. Mittheil. vol. iii (1881) 

 pp. 533-40. 



