686 



TERTIARY VOLCANIC ROCKS. 



[Ch. XXXTL 



yet "been found' in the tuffs, except impressions of the leaves of trees 

 of species not yet determined. We may confidently assume that the 

 earliest eruptions were posterior in origin to those grits and conglom- 

 erates of the freshwater formation of the Limagne which contain no 

 pebbles of volcanic rocks ; while, on the other hand, some eruptions 

 took place before the great lakes were drained, and others occurred 

 after the desiccation of those lakes, and when deep valleys had 

 already been excavated through freshwater strata. 



Fig. 728. 



Mont Perrier. 



1 2 1 2 .12 



Section from the valley of the Couze at ISTechers, through Mont Perrier and Issoire, to the 



Valley of the Allier and the Tour de Boulade, Auvergne. 

 10. Lava-current of Tartaret near its termi- 

 nation at Keekers. 

 9. Bone-bed, red sandy clay under the lava 



of Tartaret. 

 8. Bone-bed of the Tour de Boulade. 

 7. Alluvium newer than No. 6. 

 G. Alluvium with bones of hippopotamus. 

 5 c. Tracbytic breccia resembling 5 a. 

 5 5. Upper bone-bed of Perrier, gravel, &c. 

 5 a. Pumiceous breccia and conglomerate, 



angular masses of tracbyte, quartz, peb- 

 bles, &c. 



5. Lower bone-bed of Perrier, ochreous sand 

 and gravel. 



4 a. Basaltic dike. 



4. Basaltic platform. 



3. Upper freshwater beds, limestone, marl, 

 gypsum, &c. 



2. Lower freshwater formation, red clay, green 

 sand, &c. 



1. Granite. 



In the above section I have endeavored to explain the geological 

 structure of a portion of Auvergne, which I reexamined in 1843.* 

 It may convey some idea to the reader of the long and complicated 

 series of events which have occurred in that country, since the first 

 lacustrine strata (No. 2) were deposited on the granite (No. 1). The 

 changes of which we have evidence are the more striking, because 

 they imply great denudation, without there being any proofs of the 

 intervention of the sea during the whole period. It will be seen that 

 the upper freshwater beds (No. 3), once formed in a lake, must have 

 suffered great destruction before the excavation of the valleys of the 

 Couze and Allier had begun. In these freshwater beds, Lower Mio- 

 cene fossils, as described in Chapter XV., have been found. The ba- 

 saltic dike, 4', is one of many examples of the intrusion of volcanic 

 matter through the ancient freshwater beds, and may have been of 

 Miocene or Pliocene date, giving rise, when it reached the surface and 

 overflowed, to such platforms of basalt as often cap the tertiary hills 

 in Auvergne, and one of which (4) is seen on Mont Perrier. 



It not unfrequently happens that beds of gravel containing bones 

 of extinct mammalia are detected under these very ancient sheets of 

 basalt, as between No. 4 and the freshwater strata, No. 3, at a, from 



See Quart. Geol. Journ., vol. ii. p. 11. 



