690 



TERTIARY VOLCANIC ROCKS. 



[Oh. XXXII. 



the river, and in the course of a few miles rolled to sand and pebbles. 

 Meanwhile the cone of Come remains unimpaired, its loose materials 

 being protected by a dense vegetation, and the hill standing on a 

 ridge not commanded by any higher ground, so that no floods of 

 rain-water can descend upon it. There is no end to the waste which 

 the hard basalt may undergo in future, if the physical geography of 

 the country continue unchanged, no limit to the number of years 

 during which the heap of incoherent and transportable materials called 

 the Puy de Come may remain in a stationary condition. In this place, 

 therefore, we behold in the results of aqueous and atmospheric agency 

 in past times, a counterpart of what we must expect to recur in future 

 ages. 



Lava of Chaluzet. — At another point, farther down the course of 

 the Sioule, we find a second illustration of the same phenomenon in 

 the Puy Ronge, a conical hill to the north of the village of Pranal. 

 The cone is composed entirely of red and black scoriae, tuff, and vol- 

 canic bombs. On its western side, towards the village of Chaluzet, 

 there is a worn-clown crater, whence a powerful stream of lava has 

 issued, and flowed into the valley of the Sioule. The river has since 

 excavated a ravine through the lava and subjacent gneiss, to the depth 

 in some places of 400 feet. 



Fig. 729. 



a. Scoriaceous lava. 

 &. Columnar basalt. 

 c. Gravel. 



Ancient mining 

 gallery. 

 E. Pathway. 

 /. Gneiss. 



Lava-current of Chaluzet, Auvergne, near its termination.* 



On the upper part of the precipice forming the left side of this 

 ravine, we see a great mass of black and red scoriaceous lava becoming 

 more and more columnar towards its base. (See fig. 729.) Below 



Lyell and Murchison, Ed. New Phil. Journ., 1829. 



