38 The Founders of Geology LECT. 



ning of human history, they could mark the cones rising 

 one behind the other in long procession on the granite 

 ridge, each bearing its cup-shaped crater atop. 



In descending from the mountain they came upon 

 another crater, probably that of the Petit Puy de Dome, 

 a singularly perfect example of the type, some 300 feet 

 deep, and the same in diameter of rim, with such regular 

 and smooth slopes that it has been named by the shep- 

 herds the Hen's Nest. Everywhere they encountered 

 quantities of pumice, which so entirely convinced Guet- 

 tard of the true volcanic nature of the district, that he 

 found it unnecessary for his immediate purpose to ex- 

 amine the rest of the puys. Their Clermont guide, though 

 he had previously wandered over the hills, had never 

 suspected their volcanic origin; but he seems to have 

 learnt his lesson promptly, for he soon afterwards, at 

 Guettard's request, sent some details, and wrote about 

 eruptions and explosions as if he had been long familiar 

 with their effects. 



Not only did Guettard detect some sixteen or seven- 

 teen cones, but he observed that their craters looked in 

 different directions, and he thought that they probably 

 belonged to different periods of eruption. The travellers 

 pushed on to the great volcanic centre of Mont Dore. 

 But Guettard was there less successful. He was unaware 

 of the influence of long-continued denudation in altering 

 the external forms of volcanic hills, and was disposed to 

 regard his ill success as probably due to the mantle of 

 vegetation by which so much of the ground was concealed. 



The journey in Auvergne was too brief and hurried to 

 admit of any single point being fully worked out. But 



