Excavation of Valleys. 47 



natural agency, yet to this alternative will geologists, who under- 

 take such a task, be compelled to resort. 



DESCRIPTION OF THE PLATES. 



Plate I. fig 1. represents an ancient river-bed of pebbles, and sand, under 

 the lava-current of Chaluzet. C is a very ancient mining gallery 

 driven in horizontally beneath the basalt, and shews the pebble-bed to 

 be continuous between the gneiss and the volcanic rock. The river 

 Sioule is seen to be now about twenty-three feet below the level 

 which it occupied when the stream of lava flowed down this valley. 



Plate I. fig. 2. exhibits a case of much greater depth of excavation than 

 Plate I., and the junction between the ends of the prisms of basalt 

 and the gneiss is marked by a thin band of black pitchstone. This 

 lava-current flowed from the crater of Thueyts, in the Vivarais ; the 

 river is the Ardeche. 



Plate II. is a section from Issoire to Mont Perrier. The trachytic 

 breccias and conglomerates overlying alluvial beds, containing bones 

 of extinct quadrupeds, are here seen, and their relations explained, 

 to the original tertiary lacustrine formations of Auvergne. The lat- 

 ter are thus shewn to have been excavated to a considerable depth 

 prior to the operations which gave rise to the former. 



Plate III. is a transverse section of Mont Perrier, exliibiting in detail 

 the character of all the strata of trachytic conglomerates which over- 

 lie and alternate with alluvions containing fossil bones of extinct 

 quadrvipeds. The tertiary lacustrine formations beneath are seen to 

 have a dip unconformable to that of the superimposed masses. 



Errata in the preceding sheet. 



For Plate I. at page 22, read Plate I. fig. \,—And for Plate 11. at page 29, 

 read Plate I. fig. 2. 



EXPLANATION OF PLATES. 



Fig. 1. Cave at Les Combres under the lava current of Chaluzet. 

 A scoriaceous lava, passing downwards into 

 B columnar basalt. 



C sand and pebble .bed, three feet thick (water gushes out). 

 D gneiss, twenty-three feet from the pebble-bed to the level of the 



Sioule. 

 E path leading to the village. 

 l*" cave. 



Fig. 2. Lava current of Thueyts at the Gueule tPEnfer. 

 A acoriaceous lava, passing downwards into 

 B columnar basalt. 



