i Guettard's Physiographical Geology 3 1 



considers, likewise, the Rhone with its tributaries on the 

 Mediterranean side of the watershed. 1 He infers that all 

 the debris derived from the waste of the land is not carried 

 to the sea, but that a great deal of it is deposited along 

 the borders of the streams, and that though it may be 

 removed thence, this removal must require many ages to 

 accomplish. He thinks that the levels of the valleys are 

 at present being raised owing to the deposit of detritus in 

 them. 2 The plains watered by the rivers are one vast 

 sheet of gravel, the streams having changed their courses 

 again and again, so as to flow in turn over every part 

 of these alluvial tracts. The thickness of detritus brought 

 down by the rivers gradually increases towards their 

 mouths. Near their sources, on the other hand, any 

 sediment which is deposited is in a manner superficial, and 

 is liable to continued removal and transportation farther 

 down. 



The fragmentary material that is accumulated along 

 the margin of the sea is, in Guettard's view, derived either 

 from what is borne down by rivers, or from what is made 

 by the sea itself, the whole being ground into powder by 

 the long-continued beating of the waves. The sea not 

 only acts on its shores, but on submerged rocks, and the 

 detritus thus produced is mingled with the triturated 

 remains of corals, shells, fish-bones and marine plants. 3 



Comparatively little information had been gathered in 

 Guettard's time as to the condition of the sea-bottom. 

 There is thus a peculiar interest in noting the ideas which 

 he expresses on this subject. He thinks that, besides what 

 is laid down upon the shore, another portion of the detritus 



1 Pp. 225-324. 2 P. 326. 3 P. 328. 



