CONSOLIDATION OF LAGUNES. 



169 



the ordinary level of the waters ; and then come great trees which 

 solidify the soil, and attach it definitively to the continent. In the 



Fig. 70.— Mouth of the Bidassoa. 



tropical regions, it is the different species of bacbab and mangroves 

 that aid most in the formation of the new shores. Raised on the 

 scaffolding of their high roots, like aerial buttresses one above another, 

 they grow in the midst of the lagune. Hidden by the floating forest, 

 the' muddy liquid i*s soon filled with rubbish ; the branches and up- 

 rooted trunks of the trees, being much heavier than the water, in- 

 cessantly raise the bottom, and end by appearing on the surface.^ A 

 new vegetation immediately takes possession of thia yet undecided 

 shore. 



