1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 497 . 
kilometers wide, of low-level country averaging 100 m. in altitude. 
The subsoil here consists of nearly horizontal marine strata of Cretaceous 
or Tertiary age, usually hidden beneath a thick layer of clay, sand, or 
laterite. The remainder of the Lower Congo, as far east as Stanley Pool, 
is an extremely hilly country of Archzan, crystalline or older Paleozoic 
rocks, mostly sandstones or schists, in strata which have been much 
folded and tilted. We have mentioned above some of the outcrops of 
limestone occasionally encountered in the Lower Congo. Through this 
region of the ‘‘Crystal Mountains,’ so-called because of the numerous 
blocks of quartz one frequently finds strewn over the surface, the River 
Congo has gradually worn a narrow outlet to the sea, interrupted by 
numerous rapids and falls. The average altitude of the Lower Congo is 
between 100 m. and 700 m. and the highest points, such as are found in 
Mt. Bangu, hardly reach above 1,000m. The basin of the Kasai and its 
affluents, which occupies the central portion of the Southern Savanna 
District, belongs, physiographically speaking. still to the centra! basin of 
the Congo, the subsoil being formed by the horizontal, soft sandstones of 
the Lubilash system (Triassic). This area is at an average altitude of 
500 m. to 1,000 m., the highest elevations not exceeding 1,500 m. Its 
northern portion is fairly level, but in the southern part comparatively 
recent movements of the soil have rejuvenated the topography, so that 
the country is here much cut up by erosion valleys. In the eastern portion 
of the district, in Manyema and the lower Katanga (or Urua), the sub- 
soil is largely composed of crystalline rocks which have been worn down 
to a fairly uniform peneplain of about 600.m. average elevation. The 
highest point, Mt. Cleveland or Kitschima, on the upper Lualaba, in 
about 5° S., reaches 1,350 m. in height. 
The clitiate presents the same characteristics of a sharp division 
into a dry and wet season which we have described for the Ubangi 
Savanna District, but, being south of the equator, the seasons are, of 
course, inverted. The rains usually last, from September to April, but 
they frequently stop for a few weeks during January or February (little 
dry season); the dry season proper lasts from May to the end of August 
and, owing to the much greater distance of the Southern Congo Savanna, 
from the equator, the drought is more severe than in the Ubangi Savanna 
and the temperature is markedly lower. At Banana, for instance, at the 
mouth of the Congo, the total rainfall in 1912 was 59.8 inches (1, 519.] 
mm.), practically all of which was received during January, February, 
March, April, November and December. The mean temperature was 
26° .67 C.; that of the hottest month (March), 28° .91 C.; and that of 
