102 O. A. Lebom — Notes on Staurolite. 



lY. — Notes on Staurolite, in Connexion with Metamorphism. 



By G. A. Leboub, F.G.S., 



of the Geological Survey. 



IN the December Number of the Geological Magazine there is an 

 abstract of a most interesting paper published in the " Jahrbuch 

 fur Mineralogie," etc., of Leonhard and Geinitz, by C W. C. Fuchs, 

 entitled, "The Geological, Microscopical, and Chemical Examina- 

 tion of the old Sedimentary Formations in the French Pyrenees, with 

 especial reference to the Metamorphosis of these and other old 

 Eocks." In this paper attention is called to the manner in which 

 the degree of metamorjDhisin of clay-schists and other rocks is some- 

 times indicated by the formation and gradual development of certain 

 minerals, Andalusite and Chiastolite being those specially mentioned. 



Quoting from the abstract given (Geol. Mag., Vol. IX. p. 662), 

 conclusions Nos. 4 and 5 arrived at by Fuchs stand thus : "4. This 

 [metamorphism] commences with the separation of small nodules 

 in the clay-schist, which gradually [i.e. as we approach some adjacent 

 granite] increase in number and size, and at last become Andalusite 

 and Chiastolite. During the development of these minerals, the rest 

 of the rock changes bit by bit into a confused mass of mica and 

 quartz, with some felspar. 5. Lastly, real mica-schist and gneiss 

 are produced." 



Without entering into the purely chemical portion of this subject, 

 with which I could not attempt to deal, I think it may be worth 

 while to place on record a few facts which have for some years been 

 known to me, and which appear to bear out in a striking manner the 

 views just adverted to. 



In the Departments of Finistere and Morbihan it is well known to 

 tourists that on certain Pardon days, in some parishes, a brisk trade 

 is driven by retailers of religious ornaments in cruciform black 

 stones, variously known as " Pierres de croix " or " Pierres de 

 Coadry." These are simply good specimens of twin crystals of 

 Staurolite, but the cross-merchants preserve a judicious silence as to 

 the localities whence they procure them. All that one can usually 

 arrive at on the subject is that they come from someat'/tere near Coadry 

 in Finistere, or near Lhomine in Morbihan. In 1868, not being 

 satisfied with this scanty information, I made a minute and successful 

 search for the cross-stones in both places. The results were the 

 same in each case, and were as follows, taking the Coadry locality 

 only, the description being equally applicable, for the purpose in hand, 

 to it and to Lhomine. 



The geology of the part of Western Brittany in which the little 

 hamlet of Coadry is situated, namely, some eight or nine miles due 

 north of Bannalec, between Eosporden and Quimperle, consists chiefly 

 of broad tracts of clay- and mica-schists abutting against equally large 

 areas of granite, which granite, as I showed in a paper read at the 

 British Association Meeting at Exeter,' occasionally passes quite 

 gradually and imperceptibly into gneiss. The latter rock is, how- 



' Notes on aome Granites in Western Brittany. 



