234 Professor Forbes' s Sixth Letter on Glaciers. 



the fiery mouth, are radiated exactly as those of a glacier 

 under similar circumstances, and which I have represented in 

 the margin as I saw them on Vesuvius, the lines of fissure 



/ ' 



Mouth 



\ 



'^"'^'^^^^t'^.^ 



Fissures in the Crust of Lava during Crystallization. 



being marked by the liquid fire shining through. A perfect 

 analogy here exists with the phenomena of radiating fissures 

 in ice, which I first described in the glacier of the Rhone, and 

 afterwards in the ice of the Gl. du Talefre, where it joins the 

 Gl. de Lechaud, in the Gl. of Arolla, and very many other 

 instances. 



II. That the slags, where solidified, presented striae or 

 ripple-marks along their surface, parallel to the direction of 

 the " ribboned structure" of glacier ice, i. e.^ inclining slightly 

 from the sides towards the centre of the current, in the direc- 

 tion in which the current is moving. These striae, or ripple- 

 marks, which have a striking analogy in certain cases of the 

 retarded movement of rivers, are carefully to be distinguished, 

 on the one hand, from the cracks or flawsy and, on the other, 

 from the direction of motion of the fluid particles.* 



III. When, at some distance from the source, the lava be- 

 came viscid and tenacious, and forced itself, in streamlets of 

 a pasty consistence, through the interstices of its slag, thence 

 it became streaky and drawn out, in the direction last men- 

 tioned, as molten glass does in the hands of the workman. 



* A long accidental delay in the printing of this letter enables me to 

 add, that I have found in the lavas of Etna a yet far more perfect ana- 

 logy to the veined structure of glaciers than that described in the text. 

 It is, indeed, so completely developed as to leave no doubt as to the 

 identity of origin. Aug. 1844. 



