624 



GUILCHER 



[chap. 24 



because it grows more quickly. Similar mangrove successions have been de- 

 scribed as evidence of coastal changes in Malaya (Carter, 1959). It must be 

 pointed out that mangroves are not found only in muddy marshes, but occasion- 

 ally also on rocks or pebbles, and that grassy areas, resembling the temperate 

 high marshes, are found not infrequently among, or instead of, trees, for 

 example in Dahomey where mangroves are rare. On the other hand, between 

 the mangrove zone and the continental vegetation, the high -spring-tide zone 

 often remains completely free of vegetation, for exam])le in Madagascar. This 

 feature may be due to the long dry season and to the high temperature, which 



Fig. 5. Mangroves with alternating sand ridges (cheniers) in north-west Madagascar. 

 (Photo. A. Guilcher.) 



lead to complete dessication of the surface and crystallization of salt during 

 neap tides so that no plant is able to grow there. The pattern of the tidal creeks 

 in mangroves is the same as in tidal flats and marshes in temperate countries. 



C. Estuarine Mud 



In estuaries, tidal marshes and flats, the sediments consist principally of mud 

 {vase in French; Schlick in Cerman), a term which is intentionally imprecise, 

 since the sand-silt-clay ratio is extremely variable in the mud (in American 

 nomenclature, the medium grain size in clay is smaller than 3.9 [x; in silt it 

 lies between 3.9 y. and 62.5 \i; in sand it is coarser than 62.5 [x). A silt or clay 

 fraction is present in all muds, so that the mud always hardens w hen it dries, 

 whereas sand remains loose. But in some intertidal nnids the medium grain 

 size may be as coarse as 250 y., while in others it may be as small as 2,5 (i, 

 (Berthois, 1955). As a rule, the grain size is coarser in the tidal creeks, where the 



