Vol. 59.] SEDIMENTARY DEPOSITS OF SOUTHERN RHODESIA. 283 



growth. They are characteristic of the old siliceous spring-areas, 

 and can be easily distinguished from the pieces of silicified wood 

 derived from the decay of coal-seams in the Matobola Plains. 



VII. Summary. 



The geological history of Rhodesia, as set forth in the rocks under 

 notice, furnishes only a few chapters ou periods far apart. 



In the metamorphic or Archaean series that constitutes the 

 basement-formation of this part of the African continent, there is 

 evidence of basic and perhaps sedimentary rocks indurated and 

 cleaved by the lateral pressure due to intrusion of granite-masses, 

 with the resulting lines of weakness or faults rilled with vein-quartz. 

 On the principle of the deep-seated origin of granites the thickness 

 of the metamorphic rocks must have amounted to many thousands of 

 feet, a mass which has been eroded to such an extent as to lay bare 

 the intrusive granite-ridges now represented by the Matoppo Hills 

 and other mountain-ranges. 



Of the geological periods between the alteration of the meta- 

 morphic rocks and the deposition of the Sijarira Series, no repre- 

 sentative formations have been found, although in Bechuanaland and 

 in the Limpopo Valley areas of intervening rocks probably exist. 



In the north-west, after a long period of waste and decay of the 

 Archaean plateau, the land began to sink, and this depression extended 

 across the present Kalahari Desert to the west, and seemingly 

 included the area of the Karoo and Kimberley Beds on the south- 

 west. In the basin thus formed the grits resulting from the erosion 

 of the crystalline rocks of the plateau were laid down unconformably. 

 That it was a shallow sea is shown by the constantly-recurring 

 current-bedding in the 2000 feet of the Sijarira Series, and the area 

 must therefore have been one of gradual subsidence. 



As the height of the plateau was reduced by erosion, beds of finer 

 sediments became more numerous, until conditions were favourable 

 for the growth of vegetation now represented by the numerous 

 coal-seams. These beds generally lie upon hard clay, not at all 

 resembling ' seat-earth,' and no fossils have been found in the clay 

 that could be taken for roots. 



During a period intervening between the Upper and Lower 

 Matobola Beds, a sudden rush of coarser sediments (represented by 

 the Busse Beds) shallowed the water and entombed large numbers 

 of fishes, broken fragments of which can be found all along the 

 outcrop of the grits. These fishes, so far, are the oldest remains 

 discovered, and enable us to assign a Permo-Carboniferous Age to 

 the Matobola Beds. In the lagoons of Upper Matobola time, 

 molluscs of Unioniform type existed, and contemporaneous therewith 

 lived unknown vertebrates (possibly amphibia). 



Prom the base of the Escarpment- Grits coarser sediments pre- 

 dominate, and during their deposition vulcanicity became a prominent 

 factor, and ultimatelv ejected lava-flows which were interbedded with 



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