1844.] ANALOGY OF LAVA STREAMS TO GLACIERS. 45 



has any considerable breadth) into many little currents, each 

 rolling past, and being retarded by its more sluggish or already 

 consolidated neighbour ; so that its surface resembles that of the 

 bed of many torrents in the Alps, where the more solid matters, 

 the rocks, stones, gravel, sand, and clay, trace out the form of 

 a sluggish mass propelled downwards by gravity, whilst its 

 surface is seamed by the trickling of innumerable rills of water, 

 charged with the more portable materials which have been 

 washed down, or squeezed from the general mass. 



There are other circumstances, however, in which the 

 analogy of the glacier with the lava stream is more complete ; 

 and of these I will observe 



I. That the cracks of the dark-coloured slag on the surface 

 of the liquid lava, as it spreads itself abroad, on issuing from 

 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 being 



Fig. 8. 

 Fissures in the Crust of Lava during Crystallization. 



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 Glacier du Talefre, where it joins the Glacier de 

 Lechaud, in the Glacier of Arolla, and very many other in- 

 stances. 



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

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



