RHIZOPHORA MANGLE 



101 



half or three miles ; and though it thrives where the water has the 

 density of fresh- water during most of the twenty-four hours, except 

 towards high tide, when the hydrometer indicated 1-002 and 1*003, 

 it disappears when the water is permanently fresh. In the Salt 

 Springs branch of the estuary the same thing occurred. Rhizophora 

 mangle grew on the banks for the first two or three miles of the 

 ascent, when the specific gravity of the water varied between 1*000 

 and 1.003; but it disappeared altogether from the river-banks a 

 mile or two further up, where the water was permanently fresh. 

 The mode of its disappearance was remarkable, the trees not only 

 diminishing in numbers, but very markedly in size. It is thus 

 described in my journal : " Whilst ascending from Salt Springs to 

 the Blue Hole fine specimens of Rhizophora trees, fifty feet high, lined 

 the banks in places for the first half-mile. After this the trees were 

 scanty, and became smaller and smaller as we penetrated further into 

 the Great Morass, until, at about a mile and a half above Salt Springs, 

 the trees originally fifty feet in height, were reduced to shrubby, 

 sickly-looking growths, and shortly disappeared." 



Probable Infiltration of Sea-water into the Interior of 

 the Great Morass of the Black River. — The name of Salt 

 Springs, which is given to one of the principal branches of the Black 

 River estuary, would seem to indicate that salt water wells up in 

 the midst of the Great Morass in which the place thus called lies. 

 In spite of the name I could learn of no such phenomenon, but it 

 is not unlikely that sea-water does penetrate for some distance 

 through the lower portion of the dense mass of plant-growth that 

 forms the surface of the morass. Beneath the plant-growth lies a 

 platform of the rag-rock of the district, a limestone seemingly com- 

 posed in the main of old reef detritus, and not infrequently bared to 

 view. The Great Morass, the general vegetation of which is described 

 later in this chapter, extends inland for five or six miles as the crow 

 flies, and is raised but a few feet above the sea. There is some 

 ground for holding, as is explained below, that the salt water ascends 

 much farther up the river along its bottom than is indicated by the 

 density of the surface-water. But, apart from this, it would be 

 strange if the sea-soaked mangrove swamps of the coast, which are 

 physically continuous with the inland swamps of the Great Morass, 

 did not favour the landward infiltration of sea-water. 



In the case of the Machala plains on the coast of Ecuador, where 

 low-lying districts extending several miles inland constitute the sea 

 border, I have shown that this takes place on an extensive scale 

 (Plant Dispersal, p. 485). Here there is continuity in the soil-cap 

 between the mangrove belt of the coast and the arid plains miles 

 inland. The rise in level being only a few feet, the effects of the 

 sea-water infiltration are evident on the surface far from the coast 

 in a saline efflorescence on the soil. The stages of the infiltration 

 landward of sea- water are displayed : first, in the mangrove swamps 

 a mile or two in width daily overflowed by the tide ; second, in the 

 salt-encrusted mud-flats in their rear, which are overflowed only by 

 the higher tides, and support plants like Salicornia, Sesuvium, and 

 Batis maritima ; third, in the vegetated plains still farther behind, 



