102 PLANTS, SEEDS, AND CURRENTS 



which extend for miles inland, and, though sufficiently raised above 

 sea-level to be above the reach of the tides, are nevertheless soaked 

 with sea- water, that displays its presence in the salt left by evapora- 

 tion on the surface of the ground. On these plains, at a distance of 

 four miles from the coast and probably extending much farther 

 inland, there flourishes a dry jungle-type of vegetation of the xero- 

 philous kind, such as Algaroba trees (Prosopis), cactaceous plants, 

 and several sorts of prickly shrubs. 



Those who live near rivers in houses built on alluvial soil know 

 that when the river is in flood the water rises through the basements 

 of their dwellings long before the river overflows its banks. This 

 is well illustrated in the delta of the Mississippi, as at New Orleans, 

 where the graves have to be erected on the surface of water-soaked 

 ground. When low-lying districts of alluvial formation front the 

 sea, the salt water must often be found a short way below the surface 

 far inland. 



It would, therefore, be a matter for surprise if the subsoil water 

 of the low-lying region of the Great Morass in the Black River 

 district of Jamaica did not often hold in solution much saline material 

 at a distance of two or three or more miles from the sea border of 

 the swamp. But little indication of the infiltration of sea-water in 

 the subsoil might be offered by the surface-waters of the morass; 

 and mangroves, thriving away from the coast in a seemingly fresh- 

 water swamp, may actually have their roots immersed in a salt- 

 impregnated substratum. 



It is not difficult to show, as I have already done, that on the 

 banks of a river Rhizophora mangle gradually dwindles away, as one 

 ascends the stream, until it disappears altogether when the water 

 is permanently fresh. But away from the streams in the midst of 

 the Great Morass, where the surface water varies in density only 

 between 1*000 and 1*001, there occur in the Black River delta, a 

 couple of miles from the coast, isolated belts of tall Rhizophora trees, 

 fifty or sixty feet high, which seem strangely out of place in the 

 midst of a dense growth of Typha. I was told by my coloured 

 companions that these inland belts of Rhizophora are situated around 

 shallow pools infested by alligators. I could not do more than 

 examine the margin of one of these belts ; but I remarked the rela- 

 tive thinness of the leaves of the trees. They would be worth a 

 careful examination by a resident. It would seem probable that 

 salt water here wells up through the swamp, or at any rate approaches 

 the surface. 



The Underflow of Sea- water in Estuaries. — An important 

 factor in influencing the stations of plants in tropical estuaries is to 

 be found in the circumstance that whilst the surface water, more or 

 less fresh in character, is running down, there may be an under- 

 current of sea- water running up. Thus opposite Puna, at the 

 mouth of the Guayaquil River, I found on one occasion that while 

 the surface water with a density of 1*010 was running down, my 

 thermometer, when lowered to a depth of two fathoms and more, 

 was carried up-stream by a strong undercurrent, which extended to 

 the bottom at a depth of seven fathoms. Evidently the sea- water 



