Heport of the Besearches of M, AgaBiiz. 309 



rated movement of the middle portion, and that this accele- 

 rated movement, continuing throughout the whole length of 

 the glacier, the arches also are proportionally prolonged and 

 straitened, till at last they exhibit the form of very prolonged 

 ogives. 



The combination of blue bands, or bands of infiltration, with 

 the strata, is very readily recognised in the upper regions of 

 the glacier, properly so called, where their out-croppings are 

 still sensibly transverse, or but slightly convex ; for as the blue 

 bands are usually parallel to the axis of the glacier, they cross 

 the projections of the strata at different angles. But, in pro- 

 portion as these arcs are elongated, and as their sides become 

 parallel to the direction of the valley, the difficulties increase, 

 and then we can scarcely distinguish the two kinds of pheno- 

 mena, except after they have been made an object of detailed 

 study. The mode in which Professor Forbes attempts to ex- 

 plain the constitution of the glacier of the Rhone, is an excel- 

 lent proof of this. It is known that the glacier of the Rhone, 

 about half a league above its inferior extremity, presents a 

 considerable disruption — ^boulemetit, — which is known under 

 the name of the Cascade of the glacier, and which is admired 

 by all travellers as highly picturesque. Below this cascade 

 the glacier resumes its regular course, and there is seen on its 

 surface a series of lines or arcs, at first but little curved, and 

 almost transverse, which stretch out more and more, until 

 they describe semicircles, and finally the prolonged arches or 

 ogives. The crevices form a right angle with these lines, 

 whence it results that where these latter are longitudinal (on 

 the sides of the arches), the crevices are transversal ; and 

 where the lines are transversal (for example, at the anterior 

 margin of the glacier), the crevices are longitudinal. This 

 remarkable antagonism of the arched lines with the crevices, 

 of which Professor Forbes has given a sketch in the Edinburgh 

 Philosophical Journal,* is so evident, that no one can question 

 it. But the explanation which the learned Professor oflFers of 

 the phenomena appears to me altogether erroneous : He sup- 

 poses that a pressure from the descending mass operates in 



* Vol. xxjtii. p. 89, January 1842. 



