Ch. XXXIL] 



TERTIARY VOLCANIC ROCKS. 



551 



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

 and more columnar towards its base. (See fig. 611). Below this is a bed 



a. Scoriaceous lava. 



b. Columnar basalt 

 e. Gravel. 



D. Ancient mining gallery. 



E. Pathway. 

 / Gneiss. 



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



of sand and gravel 3 feet thick, evidently an ancient river-bed, now at an 

 elevation of 25 feet above the channel of the Sioule. This gravel, from 

 which water gushes out, rests upon gneiss,/, which has been eroded to 

 the depth of 25 feet at the point where the annexed view is taken. At 

 d, close to the village of Les Combres, the entrance of a gallery is seen, 

 in which lead has been worked in the gneiss. This mine shows that the 

 pebble-bed is continuous, in a horizontal direction, between the gneiss and 

 the volcanic mass. Here again it is quite evident, that, while the basalt 

 was gradually undermined and carried away by the force of running 

 water, the cone whence the lava issued escaped destruction, because it 

 stood upon a platform of gneiss several hundred feet above the level of 

 the valley in which the force of running water was exerted. 



Puy de Pariou. — The brim of the crater of the Puy de Pariou, near 

 Clermont, is so sharp, and has been so little blunted by time, that it 

 scarcely affords room to stand upon. This and other cones in an equally 

 remarkable state of integrity have stood, I conceive, uninjured, not in 

 spite of their loose porous nature, as might at first be naturally supposed, 

 but in consequence of it. No rills can collect where all the rain is in- 

 stantly absorbed by the sand and scoriae, as is remarkably the case on 

 Etna ; and nothing but a waterspout breaking directly upon the Puy de 

 Pariou could carry away a portion of the hill, so long as it is not rent or 

 engulfed by earthquakes. 



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



