Vol.63.] GEOLOGY OP THE ZAMBEZI. BASIN. 185 



possible that in this quarter the higher basalts are brought in again 

 by disturbances due to the proximity of the Deka Fault. 



Among the zeolites in my collection Mr. Thomas has recognized 

 stilbite, mesolite, and laumontite (?). Mr. F. P. Mennell also men- 

 tions scolecite as occurring abundantly in the basalt at the Falls. 1 



The chalcedonic amygdules often possess a curiously minute 

 botryoidal structure, their outer portion being made up of small 

 globules showing concentric laminae, which give the material a 

 pisolitie aspect. These and the many handsome varieties of agate 

 would make pretty ornaments if polished ; and sooner or later the 

 tourist will no doubt be able to purchase ' Zambezi Pebble ' trinkets 

 at the Falls. 



Structural Features of the Basalts. 



The apparent absence of dykes and sills in the areas which I 

 examined is noteworthy. In spite of the numerous deep sections 

 that came under scrutiny in the walls of the great gorge and its 

 branches, I saw no case where the nearly horizontal banding 

 of the series was definitely interrupted. In some places, as for 

 example at the sharp bends just below the Songwi confluence 

 (PI. XVI), the weathered precipices of the Batoka Gorge are 

 rendered jagged by the protuberance of sharp narrow spurs that 

 at first sight suggest the presence of vertical bars of harder rock ; 

 but, after as careful scrutiny as I could make by the aid of field- 

 glasses, I came to the conclusion that in this and other similar cases 

 the outstanding ridges and buttresses were simply the weathered 

 remnants of peninsulas and ' knife-edges ' like those which divide 

 the great zigzags of the river just below the Falls, and were of 

 similar origin (see p. 187). 



Small dyke-like strings, showing a vertical arrangement, were 

 occasionally visible on broad bare surfaces of basalt ; but when 

 seen in cross-section these could not be traced downward below 

 the lava-sheet on which they were displayed. They probably 

 represent merely the infilling of cracks in the consolidated crust of 

 the flow, from the yet fluid interior. On account of their differential 

 weathering, these dyke-like veins sometimes have considerable 

 influence in diverting the channels of the smaller streams ; as may 

 be well seen on a bare platform overlooking The Chasm 50 yards 

 beyond the western end of the Victoria Falls, which being occasionally 

 covered by flood-waters is sharply trenched along the course of one 

 of these veins. 



I suspected the presence of a discontinuous dyke at the truncated 

 margin of the basalts along the Deka Fault; and this suspicion 

 is partly sustained by the result of Mr. Thomas's petrographical 

 examination (see Appendix I [F 1032 & 1035], p. 207), though the 

 coarsely-crystalline rock near the Deka railway-bridge [F 1034], 

 p. 210, on which my opinion was mainly based, proves to be less 



1 Fourth Ann. Rep. Rhodesia Museum, Bulawayo, 1905, p. 2L 

 Q.J.G.S. No. 250. o 



