54 A. Milward, Esq., on the Motion of a Lava-Stream. 
When the coating of slag—for some time tolerably smooth— 
was no longer sufficiently plastic to yield to the motion of 
the viscous fluid beneath, it was torn up in fragments, which 
increased in size as the fracture occurred further down. 
The first stream which I examined illustrated this charac- 
teristic phenomenon much better than that which I have just 
described. It was a much larger stream, and presented a 
well-marked front, rising five or six feet above the cold ridge 
along which it advanced. It was rounded at its extremity, 
and advancing very slowly. Its surface presented a cover- 
ing of black irregular masses, cracked in all directions ; but 
the vertical and horizontal divisions were the most nume- 
rous. The red interior was thus very freely exposed, and 
the alteration in the position of the fragments, occasioned 
by the motion of the stream, could be readily observed. In 
places the heat had so far gained upon the coating of frag- 
ments, as to cause the surface to become red hot. As the plastic 
interior advanced, the surface and sides cracked away and 
rolled over, especially in front, where large vertical slices 
were frequently detached ; these would be more or less re- 
melted as the lava flowed on, but many of the smaller frag- 
ments roll out of the way, and line the side of the stream 
with cinders. Where the surface has remained smooth for 
a short distance, as in the small stream first described, the 
tissures caused by the motion of the interior are more clearly 
seen to depend on the tearing force exerted. A rattling 
noise was constantly audible,* but now and then a louder 
crash marked the more considerable falls. 
KEYNSHAM, SOMERSETSHIRE, 
September 1849. 
To Professor JAMESON, Edinburgh. 
* In Professor Forbes’s paper in the Phil. Trans. for 1846, p. 150, allusion 
is made to this “jostling of parts; but it is not described as a superficial phe- 
nomenon caused by the cooling of the outer parts. 
