468 Annals of the South African Museum. 



coaly shales will enclose and preserve the vegetation that grew in 

 the swamps. Elsewhere vegetable growth, being alternately wet and 

 dry, soon decays and is destroyed; casts of leaves and trunks in the 

 lighter-coloured shales and sandstones being the only evidence that 

 remains. 



Under conditions of semi-aridity, the thorough seasonal oxidation 

 of nearly all deposits except those made in permanent pools, lakes 

 etc. results in the marked dominance of deep-red and brown shales 

 and sandstones, a moderate amount of variegated shales and a few 

 containing carbon. Lime will exist disseminated in noticeable amount 

 through both shales and sandstones, and may occasionally give rise 

 to markedly nodular or solid calcareous strata. The microscope should 

 show a noticeable amount of felspar in the finer portions of the rock, 

 as well as mica. "The most marked chemical distinction of sub-arid 

 Hood-plain deposits from those of truly arid regions is found in the 

 small quantity of evaporation deposits of calcium carbonate, gypsum, 

 and salt, but especially of the two latter. Lime may be quite abundant, 

 as shown by the kankar of the Indo-Gangetic plain, its importance 

 depending largely upon the quantity in solution in the river water". 

 In river deposits of semiarid climates casts of logs are most likely 

 to be preserved in the sands deposited in the neighbourhood of stream 

 channels. Away from these channels wetting and oxidation would 

 tend to destroy the logs. 



In the ilood-plain deposits of arid climates lluvial, pruvial and aeolian 

 formations are all of wide occurrence. The most distinctive structures 

 are : (1) the presence of mud-cracks filled with aeolian sands, the 

 mud-Hakes being usually polygonal plates upturned at the edges (cf. 

 mud-Hats of Orange River, at Kheis, as described by Dr. A. W. Rogers); 

 (2) interbedding of lluvial and aeolian sands; (3) the presence of 

 scattered and facetted pebbles. 



It can be readily seen that the features shown by the Molteno 

 Beds are intermediate in character between those postulated for flood- 

 plain deposits laid down in constantly rainy climates on the one hand 

 and intermittently rainy climates on the other. The southern more 

 arenaceous facies represents the upstream portions; the northern the 

 downstream terminal beds. Du Toit has shewn that the thinning- 

 north wards to the Natal border is due to actual thinning of the 

 various members, while north of that some of the members (the 

 upper) are missing. This is what would be expected from the sup- 

 position of a land to the south contributing rock-waste which was 

 laid down in the form of a fan at the foot of the mountains, the 

 amount of material becoming less as time progressed. The absence 



