450 Late Explorations in the Gaboon. [June, 
their origin to the sand heaped up along the shore by the very 
strong surf, we find, among transported stones, roundish masses 
of quartz which, when broken, show the most beautiful quartz 
crystals. 
The effect of the tides is felt in the labyrinthine estuary of the 
lower river districts as far as forty miles into the interior. Here 
the tides act like a dam checking the flow of the rivers, which in 
the rainy season rise four or five meters, and thus all the adjacent 
country for many miles is inundated. 
With the exception of a few higher points where forest trees 
grow, the vegetation of this swampy lowland consists mostly of 
thickets of mangrove, from whence comes, under the stimulus of 
the tropical sun, the aria cativa (malaria) so fatal to Europeans. 
It is extremely dangerous to travel through the districts where the 
air is impregnated with poisonous exhalations, and efficient proph- 
ylactic measures must be taken if one would escape. It has 
been my custom to take for this purpose one or two doses daily 
of quinine, and to wear a respirator of fine wire gauze over my 
mouth. In the evening I bathe the entire body with salicylic 
acid mixed with a few drops of spirits of ammonia and ol 
glycerine. 
Further up the stream, and beyond the mangroves, and where 
the water is less saline, we find the Pandanus, the wild date 
palm (Phenix spinosa) and the trunkless Raphia, which forms 
tangled thickets. All these plants, as well as the Bombax ( Erto- 
dendron anfractuosum), though the latter least, endure the brack- 
ish water, but grow quite as well where the water saturating the 
soil is fresh. On banks which are not reached by the high water, 
the Bombax rises to a respectable height, and can be seen from 
a great distance overtopping all the associated plants. This tree 
throws out strong buttresses which reach up to one-third of the 
height of the trunk, protecting it from fracture to which its tem- 
der wood would be exposed as well as from being uprooted. The 
above mentioned palm trees and Pandanus form the foreground 
to the forest of tall trees which covers the highlands of the inte- 
rior. In this zone the mangrove is replaced in the shallow water 
by a species of papyrus which sometimes covers large tracts. In 
time of flood, matted masses of it many rods square are lifted 
from their anchorage and carried even into the ocean, where they 
often surprise and mislead navigators, 
