80 



ALLUVIUM IN AUVERGNE. 



[Ch. VII. 



able connection with the agency of floating ice and glaciers will be treated 

 of more particularly in the eleventh and twelfth chapters. 



The student will be prepared, by what I have said in the last chaptei 

 on denudation, to hear that loose gravel and sand are often met with, 

 not only on the low grounds bordering rivers, but also at various points 

 on the sides or even summits of mountains. For, in the course of those 

 changes in physical geography which may take place during the gradual 

 emergence of the bottom of the sea and its conversion into dry land, 

 any spot may either have been a sunken reef, or a bay, or estuary, or 

 sea-shore, or the bed of a river. The drainage, moreover, may have been 

 deranged again and again by earthquakes, during which temporary lakes 

 are caused by landslips, and partial deluges occasioned by the bursting 

 of the barriers of such lakes. For this reason it would be unreason- 

 able to hope that we should ever be able to account for all the alluvial 

 phenomena of each particular country, seeing that the causes of their 

 origin are so various. Besides, the last operations of water have a 

 tendency to disturb and confound together all pre-existing alluviums. 

 Hence we are always in danger of regarding as the work of a single 

 era, and the effect of one cause, what has in reality been the result of a 

 variety of distinct agents, during a long succession of geological epochs. 

 Much useful instruction may therefore be gained from the exploration of 

 a country like Auvergne, where the superficial gravel of very different 

 eras happens to have been preserved by sheets of lava, which were 

 poured out one after the other at periods when the denudation, and 

 probably the upheaval, of rocks were in progress. That region had al- 

 ready acquired in some degree its present configuration before any volca- 

 noes were in activity, and before any igneous matter was superimposed 

 upon the granitic and fossiliferous formations. The pebbles therefore in 

 the older gravels are exclusively constituted of granite and other aborigi- 

 nal rocks ; and afterwards, when volcanic vents burst forth into eruption, 



Fig. 99 



Lavas of Auvergne resting on alluviums of different ages. 



those earlier alluviums were covered by streams of lava, which protected 

 them from intermixture with gravel of subsequent date. In the course 

 of ages, a new system of valleys was excavated, so that the rivers ran 

 at lower levels than those at which the first alluviums and sheets of lava 

 were formed. When, therefore, fresh eruptions gave rise to new lava, 

 the melted matter was poured out over lower grounds ; and the gravel 



