32 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
in the washing of the sides of the valley by the scorice to a considerable height 
on either hand ; (2.) from the nicety with which it has filled every angle of this 
irregular channel ; (3.) from its almost total disappearance for several miles from 
its course, shewing (I apprehend) that it ran over the ground almost without con- 
solidating. 
Feet. 
The Height of the Pont de la Beaume above the Sea is : é 1075 
Village of Burzet* ... be : : 1815 
742 
Rise of the Valley in 8 miles, A ; ; P 
Mean Slope (1 in 57, or) 1° 0’ 
At the crater of Fiollonge the barometer stood, on the 26th June 1841, at 
670°7 millimetres. Its height is 3678 English feet. If we deduct from this 
300 feet, as a rough estimate of the height of the crater above the bed of the 
river of Burzet, at the foot of the cascade of Raipis, we have for the rise in 
the upper 8 miles of the valley from the village of Burzet, 1563 feet. 
Mean Slope (1 in 27, or) 2° 7’ 
Even on my first visit in 1839, I was so much struck by the fluidity which 
this lava stream must have possessed, that I made some experiments during the 
following winter, with the kind permission and aid of the late Mr Epryeron of 
Glasgow, upon the flow of cast-iron in very small and tortuous channels at 
different slopes. The results are scarcely worthy of minute detail ; but they left 
the impression on my mind that the temperature and the liquidity of the lava of 
Burzet must have at least equalled that of pure cast-iron, a result which strik- 
ingly contrasts with the extreme viscidity of most modern lavas, which only attain 
a long course, aided by very steep inclinations, or by their great volume, which 
generates a vast moving power as well as retains the high temperature. I found 
that in tortuous channels of half-an-inch broad, and under slopes of about 1 in 
120, the hottest iron used in ordinary casting solidified before it had run a course 
of many feet. The form of the current, too, was the same as in the lava; the sur- 
face was concave, except near the lower termination of the stream, where it be- 
came convex, owing to the pressure from behind, and the accumulation due to the 
increasing friction occasioned by the crystallization of the iron which appeared to 
be rather suddenly effected. Near the source a mere ¢rail of slag was left (often 
hollow), and the slag bathed the sides of the channel to some height, as described 
in the case of the lava. 
In making these experiments, I little thought that in a few years I should 
have occasion to return to the subject of the motion of viscous fluids in narrow 
channels, in connection with the seemingly opposite subject of glaciers; but, in 
fact, the forms of my cast-iron models (which I still possess), recall strikingly 
* Barom. 715:6 mm. 26th June 1841. 

