198 THIRTEENTH LETTER ON GLACIERS. [1846. 



central part, between the two medial moraines at its issue into 

 the valley from between the colossal summits of the Aiguilles 

 Rouges and Mont Broglia ; in fact, at a point near the letter r 

 of the word " Glacier " on the eye-sketch of a ground-plan of 

 the glacier in my published volume. From the 7th to the 10th 

 August, I found the daily motion to be 9'9 inches ; or the very 

 same as that just given for the Glacier de Nant Blanc, than 

 which, two cannot be more dissimilar. But the smallness of 

 the motion in the latter case is due to the great height and 

 very small nM or reservoir ; in the former, to the exceedingly 

 slight declivity and enormous frontal resistance to the exit of 

 the glacier into the Allee Blanche, already encumbered with its 

 ponderous mass and debris. 



Whilst speaking of the Glacier du Miage, I may add, that 

 I found on the most westerly of the medial moraines, dark blue 

 lias-schist, which undoubtedly comes from the very axis of this 

 granitic chain, where it has not hitherto been suspected to exist. 

 Also, that a repeated examination of the curious " ancient mo- 

 raines," forming semicircular embankments towards the Lake of 

 Combal, described in my Travels, p. 194, and figured in the 

 eye-sketch, has convinced me that these cannot be of so old a 

 date as I then believed ; but are due to a temporary outbreak 

 of the glacier at some not very remote period, when the accu- 

 mulation of pressure of the ice has been such, that an overflow 

 took place laterally, which deposited these moraines in succes- 

 sion as it retired, and the glacier retreated to its present limit. 

 I found an exactly analogous case in the Glacier des Bois, at 

 Chamouni ; which, at some period not on record, has thrown 

 out an arm between the Chapeau and the Cote du Piget. 



Having fully described the measures of the Glacier of La 

 Brenva in my last letter, I shall not here return to them. 



Thus I have been able to add the velocity of four additional 

 glaciers to the small number previously ascertained. 



