Professor Forbes's Sixth Letter on Glaciers. 233 



of volcanic projectiles. The lava issued in a very steady rapid 

 stream, and spread itself over a gentle declivity with a velo- 

 city of not less, I think, than a foot per second. 



Admitting the plastic or viscous theory of glaciers, the 

 resemblance to lava fails (1.), In respect of the great liquidity 

 of the lava near its source ; (2.), From its very unequal rate of 

 consolidation ; a crust being very soon formed upon the sur- 

 face, which becoming more and more massive, the principle of 

 fluidity is not uniformly distributed throughout the mass, as 

 in the glacier, but a tolerably perfect fluid struggles with the 

 increasing load of its ponderous crust, which it tears and rends 

 by the mighty energy of hydrostatic pressure ; and here and 

 there finding a freer exit far removed from its source, tosses 

 high those mighty fragments of the stony arch which confined 

 it into the wild shapes which strike the eye in crossing the 

 wastes of a lava stream, and which seem at first incompatible 

 with the fluid or semifluid principle of motion. This second 

 circumstance, then, — the very unequal and rapid superficial 

 consolidation of the lava near its source, — has no analogy in a 

 glacier, nor even in a river, unless when breaking up a pon- 

 derous crust of ice after a sudden thaw. The regulated pro- 

 gression of the glacier, swiftest in its centre, and with a gra- 

 duated retardation towards the sides, has a much more precise 

 analogy to that of a river than the lava stream has, which 

 is subdivided (when it 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 ana- 

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

 and of these I shall 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 



