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Twelfth Letter on Glaciers. Addressed to Professor Jameson. 

 By Professor J. D. Forbes.* 



On the Extraordinary Increase of the Glacier of La 

 Brenva from 1842 to 1846 — its Motion — Experimentum 

 crucis respecting the Origin of the " Veined Structurey 



My dear Sir, — In continuation of tlie results of my re- 

 cent journey, of which I communicated a part in my Eleventh 

 Letter, I shall first give some account of the phenomena 

 observed on a fresh visit to the glacier of La Brenva, on the 

 south side of Mont Blanc. Having undertaken the journey 

 from Chamouni to Courmayeur and back, for the sole pur- 

 pose of examining anew this glacier and that of Miage (of 

 which I have already given a full account in the tenth chap- 

 ter of my Travels in the Alps of Savoi/, &c.), I thought my- 

 self fortunate in obtaining some important and unexpected 

 observations, as well as in again meeting M. Carrel, canon 

 of Aoste, who, in company with M. L'Eglise, canon of the 

 Great St Bernard, very kindly made a visit to Courmayeur, 

 on purpose to join me, and afterwards accompanied me upon 

 the glaciers. 



Upon arriving opposite to the glacier of La Brenva, on the 

 6th August last, in descending from the Col de la Seigne, I 

 was first struck by the surprising extension of volume which 

 it had undergone since my last visit in 1842. This will be 

 understood by a comparison of the limit of the glacier in 1842, 

 as sketched in the plan No. XL, opposite to page 193 of the 

 first edition of my Travels, and reproduced in Plate 11. , fig. 1, 

 accompanying this paper, and the line of its extension in 

 1846, marked by a dotted line in the same figure. I wish it to 

 be understood that the sketches in question being only taken 

 by the eye, have no pretension to exact accuracy, but fortu- 

 nately the land-marks are sufficiently distinct to prevent the 

 least dubiety. Thus, for example, the bay or hollow oppo- 

 site to C in the sketch, and which was drawn, in 1842, as 

 filled with the old moraine of 1818, was filled up with ice 



* Read to the Royal Society of Edinburgh, 7th December 1846. 



