186 ME, G. W. LAMPLUGH OX THE [May I907, 



dyke-like than I anticipated. As trap-like characters reappear in the 

 basalts within a few yards of the fanlt in several places, the dyke, 

 if it 'exists, can be of no great breadth. The so-called ' Mavinga 

 Dyke ' shown on an engineer's sketch-map of the Wankie coalfield 

 (see ante, p. 173 & footnote), though it happens to fall in the same 

 position as this supposed intrusion, represents a misapprehension 

 of the faulted junction of the basalts and sandstones, as it was 

 evidently intended to include all the basalts seen beyond the fault. 



Dips. 



In a series of lava-flows the true stratigraphy is necessarily 

 difficult to decipher, as the structural dip may be completely masked 

 by their original inclination. Hence it is doubtful how far the 

 dips that were observed in the Batoka Basalts are original, and 

 how far superinduced. One thing is certain, that the surface of 

 the plateau does not stand in simple relation with the original 

 surface of the lava-field, but has been developed by erosion across 

 the edges of the flows. 



The dips observed along our route are recorded, with other local 

 information regarding the basalts, in tabular form in Appendix II 

 (p. 212), which contains the details condensed from my note-book. 

 From this it will be noticed that gentle south-easterly dips prevail for 

 a few miles eastward from the Falls ; and that this direction is 

 reversed for a space in the middle portion of the district, but reappears 

 occasionally towards the eastern limit of the route. The dips, how- 

 ever, are usually so low, that it would be unsafe to assign much 

 significance to them ; and in the few restricted areas where com- 

 paratively steep dips were observed, they might well be due to 

 original irregularities. Nevertheless, it is almost certain that 

 structural tilting of some degree would result from a dislocation of 

 the magnitude of the Deka Fault ; and the evidence as a whole 

 suggests that the dominant dip of the basalts is southward or south- 

 eastward from the core of ancient rocks forming the Batoka upland, 

 until they are intercepted by the great fault. On a broad scale, 

 therefore, the basalts may be regarded as occupying a faulted 

 syncline. 



Joints and other Fractures. 



The basalts are everywhere strongly and closely jointed, and the 

 joint-system possesses a remarkable uniformity of direction over 

 very wide areas. The prevalent direction of the governing set is 

 approximately east and west, usually a few degrees — say 10° to 20° — 

 north of east and south of west. 1 There is also a tendency for other 

 joints to be developed more or less at right angles to this set, which 

 are however much less regular and in every way less conspicuous 

 (see Appendix II, p. 212). 



As one might expect, the joints, especially the governing set, 



1 From travellers' descriptions of the country north and north-west of the 

 district here described, it may be gathered that east-to-west jointing and 

 faulting is extensively developed in this part also. 



