684 SYDNEY H. BALL AND MILLARD K. SHALER 



mashed and recrystallized into gneisses. Large bodies of granite 

 and smaller ones of diabase also occur. 



To the east of these older rocks are limestones and calcareous 

 schists, which near the ancient complex are closely folded, but 

 are flat-lying farther east, half-way between Matadi and Leopold- 

 ville. These beds are considered by E. Dupont^ to be of Devonian 

 age. To the east they are unconformably overlain by red sand- 

 stones and shales, the Kundulungu (Permo-Carboniferous) of 

 Professor Jules Cornet.^ 



The great interior region is covered by the Lubilache formation, 

 a series of interbedded sandstones and shales, either flat-lying 

 or dipping gently toward the center of the basin. The formation, 

 the lower surface of which is undulating, thins rapidly on the upper 

 slopes of the basin. As the rim of the basin is approached, isolated 

 inliers of older folded and faulted rocks begin to appear where val- 

 leys cut through the sandstone-shale blanket, and encounter low 

 domes on the old surface. 



These older rocks are similar, in a general way, to those of the 

 Crystal Mountains. They consist of very ancient mica schists, 

 quartzites, and igneous schists and gneisses, which are cut by 

 gabbros and granites. Younger than the granites are folded 

 sandstones, quartzites, slates, shales, and limestones. Cornet 

 is, without much question, correct in his belief that these two series 

 of rocks are respectively of pre-Cambrian and Paleozoic age, 

 although, unfortunately fossils have nowhere been found in them. 

 In the Eastern Mountain area are also modern and Tertiary 

 lavas. 



The main structural features on the north and south rims of 

 the Congo basin have an east-and-west trend, while the rocks of 

 the Eastern Mountains strike north-and-south. The position of 

 Lakes Tanganyika, Kivu, Albert Edward, and Albert and of the 

 mountain ranges near by has been determined largely by faults, 

 the trends of which are approximately parallel to the strike of the 

 rocks. 



^ Lettres sur le Congo (1889), Paris. 



* Bulletin Societe Geologique de Belgique, XXI, 262. 



