466 Bulletin American Museum of Natural History [Vol. LIII 
can hardly sustain snail-life at all and, as it covers very great areas of the 
Congo, its predominance goes far to explain the general poverty of 
mollusean life in these regions.! 
Outcrops of older rocks form locally residual hills (‘‘Inselberge’’) 
of granite and gneiss, as in the peneplains of the northeastern Congo 
(Pl. LIV, fig. 1), dykes and ridges of quartz, as in the Crystal Moun- 
tains of the Lower Congo, rocky ledges in river beds, especially in the 
neighborhood of the numerous falls and rapids, or cliffs bordering 
certain ravines. Limestone or rocks that are rich in lime are rarely met 
with at or near the surface. They are perhaps more frequently found 
in the Lower Congo, between Matadi and Leopoldville, in the region 
occupied by Cornet’s Paleozoic “Schistocaleareous System.” This 
aggregate of strata consists for a large part of pinkish and gray 
dolomites, odlithic limestones, sandstones, schists, argillaceous lime- 
stones, etc. The limestones often form prominent landmarks or pic- 
turesque cliffs, such as the rocks of the Zole Pass, of the ‘‘ Montagnes de 
Marbre,” of Bafu, of Lamba, of Dia Bavo, of Mt. Kinsundi, and the 
magnificent cliffs of white, gray, and blue marbles at the left of the 
old caravan road between Nsona Kibaka and Lukungu. A number of 
caves are excavated in these calcareous rocks, notably in the region of 
Thysville. All these limestone outcrops should be explored malacologi- 
cally at the proper season and may furnish interesting discoveries. 
Similar limestone rocks are also present in southern Katanga, 
where the so-called “Kambove Beds” form an alternating series of dark 
to light dolomites, sandstones and shales, but the country here has 
passed through a much more complete cycle of erosion so that it is now 
worn down to a uniform peneplain, with few outcrops of unaltered bed- 
rock. “Sinks,” where running streams disappear into underground 
channels, and caves are sometimes found in the Katanga dolomites and 
such localities may be worth investigating by the malacologist.2 Along 
the Aruwimi-Ituri River there are at a few points outcrops of a compact, 
white limestone, usually in rapids or at the shore, plunging directly into 
the waters of the stream. The best-known are those of Mopele (between 
Bomili and Avakubi) and those a few hours above Avakubi. Others, 
of small extent, have been noted at various points of the Upper Uele 
(1919, Bull. Soc. Path. Exot. Paris, XII, pp. 238-243), for instance, attributes to the low content of 
oe in the natural waters of the Belgian Congo the prevalence in that territory of osteoporosis one 
e equines. : 
*See Studt’s map for the location of some of the outcrops of limestone in Upper Katanga. (F. E 
eta 1908, ‘Carte géologique du Katanga.’ Ann. Musée du Congo, Géol., Sér. II, pt. 1, pp. 5-16, 
ap. ’ 
