228 THE LIFE OF JAMES J). FORBES. [CHAP. 



slowly on to the Puy de Dome, which I ascended from 

 the Petit Puy. . . . 



* Mem. Decomposed granite ia sometimes hardly 

 distinguishable from tuff, and domite has nearly the 

 same ingredients : I have no doubt that it is an altered 

 granite. . . . The view from the top, which is easy of 

 ascent, and not so high above the plateau as it looks, 

 is very noble, and I admire more and more the situation 

 of Clermont. The form of the mountain is that of a 

 parabola squared down at the vertex, and its top has 

 somewhat of a crater aspect, but it has obviously been 

 artificially dealt with. A chapel built of quarried basalt 

 and scoriae formerly existed. 



4 1 descended by the valley of Eoyat, which I was 

 rather too fatigued to enjoy, as I had not recovered from 

 my cold and the fatigue of Mont Dor. I reached Cler- 

 mont at five/ 



Journal, Sept. 22. 



' I left Pontgibaud under the guidance of my innkeeper, 

 to see the Fontaine d'Oule, a hollow in the great lava 

 stream which contains ice in summer, and none in 

 winter. A guide was most necessary, not only to find 

 the place amidst that scene of desolation, but when 

 found to find the ice, the season being so far ad- 

 vanced that scarcely any remained ; but a piece " large 

 enough to swear by" was at length discovered. JVly 

 theory of this is simple. There are notably many parts 

 of the cheire, or lava stream, where snow lies in the 

 hollows during most of the year. The cheire being 

 composed here almost entirely of detached blocks, I 

 presume that the melting of the said masses of snow pro- 

 duces currents of water at 32, which, dropping through 

 this grotto at a spot entirely sheltered from the rays of 

 the sun, may readily by its own evaporation be suffi- 

 ciently cool to render it solid. It is obvious that this 

 process can only go on while the ice is melting, i.e. 

 in spring and summer ; in winter, as the source is frozen, 

 there is no further accumulation of ice. 



