134 dr. f. Oswald on the [June 1914,. 



green anxphibolite l belonging to the basement-floor of schists and 

 gneisses, upon which these Miocene beds were deposited. (See 

 §IV,p.l38.) 



Summary of Physical Conditions. 



The quartz-ironstone breccia was probably derived from the- 

 weathered products of the old land-surface, which must have 

 accumulated to a great thickness before the transgression of 

 the lake formed the lagoon in which the Miocene Series was 

 deposited. Therewas doubtless a more or less steady subsidence 

 throughout the period of deposition, rapid at first and gradually 

 diminishing in intensity. To these earth-movements may be 

 ascribed the formation of the quartz-veins which traversed not 

 only the basement of amphibolites, but even penetrated for a 

 short distance into the lowest clays (at Nira), accompanied by 

 ferruginous thermal waters reddening the basal clays. Calcareous 

 springs were also active, especially in the lower section of the 

 deposits, but travertine was formed intermittently up to the top of 

 the whole series. The torrential character of the sediments brought 

 down by a large river (probably the precursor of the present Kuja) 

 is prevalent in the lower series, as shown by the frequent alternation 

 of current-bedded sandstones with beds of gravel at Kachuku; but 

 farther west, at Nira, the older part of the same period is represented 

 by a prevalence of clays and marlstones, while on the east the 

 whole series is missing. The upper part of the lower series is 

 marked by the formation of beds of calcareous nodules, showing 

 the existence of fairly strong currents, capable of rolling along and 

 keeping in constant movement large and heavy nodules up to 2 feet 

 in diameter in the lime-laden waters of the lagoon or shallow gulf. 

 This was followed by a well-marked torrential period of calcareous 

 gravels, the constituents of which were derived, not from the- 

 south, but from the east. 



The middle series marks a transitional period when the new 

 river-system was becoming mature, wherefore torrential deposits are- 

 exceptional and temporary, forming only thin beds of fine gravel. 

 White or pale-grey current-bedded sandstones are characteristic of 

 the commencement of this series, but there is a continually increas- 

 ing tendency to deposit finer and yet finer sediments until red clays 

 predominate, interrupted by travertinous layers and more rarely 

 by thin seams of sandstone. The remarkable persistence of the 

 travertinous marlstone of No. 14, which is readily recognizable- 

 from Nira to Kikongo (where, however, it has lost its redness 

 and has become quite grey), seems to indicate the presence at this 



1 The rock is a fine-grained felt of green acicular hornblende, abundant 

 microlites of an acid labradorite, quite fresh and doubtless secondary,, 

 and fine aggregates of small quartz-granules. A few unaltered grains of 

 ilmenite still occur, although it has mostly been changed to leucoxene, 

 and there is a little zoisite and diffused calcite. The original rock was, 

 not improbably, a dolerite. 



