1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 465 
scarcity of terrestrial species he attributes chiefly to the yearly grass 
fires which restrict-mollusks to certain protected areas, notably to the 
woods. Since, however, these animals are not, generally speaking, much 
more abundant in the African lowland rain forest than in the savanna, 
we fail to see how grass fires alone can account for the poverty of the 
fauna, although we fully recognize the influence of this factor. Un- 
doubtedly, several ecological conditions concur to render our territory 
unfavorable for molluscan life, but for the present we are inclined to 
regard the extreme scarcity of lime in the soil as of foremost importance. 
A brief discussion of soil conditions in the Congo seems therefore quite 
in order. a 
The subsoil of the Belgian Congo consists of a base of old crystalline 
or metamorphic rocks of Archzean or early Paleozoic age. Toward the 
periphery of the Congo basin these older rocks come near the surface 
either as massive blocks or as much distorted, folded, and tilted strata. 
At one time they formed there mountain ranges which are now, however, 
much denuded and worn down to moderately high ridges or to extensive 
peneplains. Only in Katanga and in the highlands along the eastern 
border of the Belgian Congo are the outcrops of older rocks more rugged, 
due to recent fault movements of considerable amplitude in the region 
of the Albertine Rift. In the central, flattened, and more or less bowl- 
shaped portion of the Congo basin, the older rocks are buried beneath 
great thicknesses of horizontal sandstones and shales of continental or 
lacustrine origin and mostly of Permo-Triassic age, which cover about 
three-quarters of our territory. Similar non-marine strata also form 
the high plateaus of Katanga. In a narrow strip, not much over twenty 
miles wide, along the Atlantic coast, there are marine deposits of Creta- 
ceous or early Tertiary age. Whenever exposed to the meteoric agents, 
the weathering of all these rocks is extremely rapid under the combined 
action of heavy rains and uniformly high temperature, so that the bed- 
rock is as a rule hidden beneath thick layers of detrital products. Accord- 
ing to the nature of the bed-rock, alteration in situ will produce either 
sand or clay and frequently leads to that peculiar, much leached tropical 
surface soil known as “‘laterite,’’ which covers much of the country 
in the northern and southern portions of our territory. Laterite is a 
rock of somewhat variable composition, usually a mixture of hydrated 
oxids of iron, which give it a reddish or brown color, and hydroxids of 
aluminum, titanium, and rarely manganese. It is a residual accumula- 
tion resulting from the removal in solution from the rocks affected of 
combined silica, lime, magnesia, soda, and potash. Such soil, of course, 
