Vol. 59.] THE MAGNETITE-MINES NEAK COGNE. 55 



7. The Magnetite-Mines near Cogne (Geaian Alps). By Prof. 

 T. G. Bonnet, D.Sc, LL.D., F.R.S., F.G.S. (Read December 

 17th, 1902.) 



Though the magnetite-mines of Cogne in the Graian Alps are 

 frequently mentioned, I have not succeeded in finding any detailed 

 description of the relation of the ore to the adjacent rocks ; so, as 

 this seems to me interesting, I venture to publish a few notes made 

 during the past summer when, in company with my friend the 

 Rev. Edwin Hill, F.G.S., I spent some little time at the Hotel de 

 la Grivola in that village, where I had already twice halted for a 

 night or two in my climbing days about forty years ago. These 

 mines are said to have supplied ore to the Romans, and have 

 certainly been worked since the beginning of the fourteenth 

 century — though for many years past little or nothing has been 

 done, not because of any failure in the ore, which is practically 

 inexhaustible, but owing to the cost of placing it on the market ; 

 for though Aosta is now accessible by railway, it is between five 

 and six hours' walk from that town to Cogne, and the road after 

 the first 5 miles is only fitted for charrettes (country-carts). 

 The mines, also, are not at Cogne, but high up on La Ruine Blanche, 

 a spur of the Pointe de la Creia : three workings (according to my 

 sheet of the Quadro di Unione, 1852) being on its southern face 

 above the main valley, and one on its western flank above the 

 Vallon de Grauson. After visiting the last-named and the most 

 important of the others, we can well understand why they have 

 been abandoned. 



The Filon de Licone, or Le Grand Pilon — one of the former 

 group — and not much less than 2J hours of steady walking from 

 Cogne (5033 feet), is 7667 feet above sea-level, by a rough mule- 

 path ; the other, the Pilon de Larsine, is at a lower level, and 

 nearer Cogne 1 ; but part of the more direct path, owing to a fall of 

 rock, is now in a very bad condition. The mountains on the side 

 of the Val de Cogne consist of calc-mica-schists, hornblendic schists, 

 and serpentines : the first varying from almost pure marbles (not 

 common near Cogne) to rather conspicuously micaceous schists ; 

 the second (Griiner S chief er), also rather variable, are generally 

 fine-grained, of a distinctly green colour and schistose, but are 

 sometimes more granular and dioritic. The serpentines I shall 

 presently notice. 



According to Cavaliere W. P. Jervis, 2 the magnetite forms 



' un filone strato incassato tra il calcare bianco-giallognolo e gli schisti talcosi 

 della zona delle pietre verdi ; in cui sono numerose vene di minerale, divise 

 da banchi di roccia serpen tinosa.' 



3 I forgot to read my aneroid, and we did not walk direct to the mine from 

 Cogne ; but I should think that the height was not far from 6000 feet, and 

 the distance by time about one half that of the other place. 



2 ' I Tesori sotterranei dell' Italia' pt. i (1873) § 211, 



P 



92. 



