SEDIMENTARY ROCKS OK RHODESIA. 



353 



as a sign of intrusion. There is no evidence anywhere of trans- 

 gression across the bedding of the sandstones^ and it accord- 

 ingly seems probal)le that even the non-vesicular Ijasic rocks are 

 true lava flows. Deposits of laterite often result from alteration 

 of thj lavas. They are usually highly ferruginous, and seem 

 rarely to contain any considerable amount of alumina. 



DiSTRii'.UTiox.— The various tracts of " Gusu " on the 

 plateau are nearly always situated on the elevations forming 

 the watersheds between the main streams, and at onlv one 

 point, namely, near the Leopard road on the Vungu River, do 

 1 know of an instance in wdiich an important watercourse is 

 crossed, until one enters the country to the north, where no 

 other rocks are exposed. This feature is well brought out by 

 mapping, as will be seen from an inspection of the ma.p on 

 page T,^2. On crossing such a river as the Bembezi or Shan- 

 gani, the Forest Sandstone areas present the appearance of long. 

 heavily wo(xled ridges with i^erfectly flat tops. At a distance 

 they have a dece])tive aspect of steepness, but the slopes are 

 usually so gentle that it is difficult in travelling to tell whether 

 one is going up or down. There are several small outliers near 

 Bulawayo, of which the largest is marked by the conspicuou.^ 

 basalt-capped hill of Taba-z-Induna. twelve miles from the 

 town. Another is found north of Inyati Mission Station. The 

 Somabula Forest marks a typical long strip stretching along 

 the (Jwelo-Shangani Watershed right on to the apex of the 

 plateau south of the town of Gwelo. Its southern extremity 

 is marked by a series of gravels and sands whose relation 

 to the more typical forest sandstones is uncertain, though thev 

 are unquestionably younger. From them has. been extracted 

 some quantity of diamonds and other gems. 



Physioor.\phv. — We have alread}- remarked on how the 

 narrower tracts of Forest Sa-ndstone country are confined to the 

 watersheds between the larger streams which run towards the 

 Zambezi. This point, together with a consideration of the 

 levels, is sufficient to show that the metamorphic area oi the 

 ])lateau represents the original plain of denudation on which the 

 sandstones were laid down. It has only begun to be dissected 

 by the present-day river .systems, a fact which accounts for the 

 want of striking features near Bulawayo and along the line of 

 railway which follows almost exactly the axis of the plateau right 

 through Matabeleland. Successions of vleis usually mark the 

 edges of the Sandstone tracts. It is very interesting to observe 

 that there are still certain pre-Forest Sandstone features to be 

 discerned. There is a dolerite dyke, which illustrates this point 

 well, which forms a long, rocky ridge by the Leopard road near 

 the Shangani River. This disappears under the sand on the 

 rising ground near the ATmgu, but emerges quite unexpectedly 

 as a ridge ju.st north of that river, together with the granite in 

 which it is intrusive. The Banded Ironstones show similar 

 features very ]:)rominently. Thus they protrude through the 



