184 ME. G. W. LAMPLUGH ON THE [May I907, 



investigation may yield more notable results than I found myself 

 able to attain. 



Some of the finer breccias present tuff-like characters ; and though 

 it was my impression in the field that all the fragmental rocks that 

 came under my notice were ' flow-breccias ' derived from the surface- 

 shattering of the lava-streams, the petrographical examination of 

 my specimens has shown that in at least one case, the composition 

 of the rock cannot be entirely thus explained. A finely-fragmental 

 rock collected from a thin band among hard platy basalts at the 

 Mavangu creek, about 17 miles east of the Victoria Falls, has been 

 found by Mr. Thomas to contain much clastic quartz ; which denotes 

 either that the bed is a true tuff, or that it represents an admixture 

 of land-waste with the basaltic detritus (see Appendix I [F 1018], 

 p. 210). 



It is to be remembered that Mr. Molyneux l and Mr. Mennell 2 

 have described the intercalation of basaltic lavas with the ' Forest 

 Sandstones ' of the country to the eastward, between Bulawayo and 

 the Zambezi ; and that these lavas are assigned to the same period of 

 volcanic activity as the Batoka Basalts — with which indeed they 

 may at one time have been directly connected, though now severed 

 by the Deka Fault. If this correlation be established, it will imply 

 that sandy deposits of wide extent were accumulating contempo- 

 raneously around the outer margin of the Batoka lava-field and 

 were occasionally interbedded with the fringe of the flows ; and 

 the sand-grains in the Mavangu breccia may be due to wind-drift 

 from some such marginal area. 



The Amygdaloids. 



The basalts are prevalently amygdaloidal, sometimes only slightly 

 and sometimes highly so, the cavities being filled sometimes with 

 green-coated chalcedony or crystalline quartz or a combination of 

 both, and sometimes with zeolites. The amygdules vary in size 

 from a mere speck to 8 or 10 inches in diameter ; and I noticed a few 

 isolated instances in which they assumed a pipe- or tube-like form. 

 Around the Falls and in the higher part of the Batoka Gorge, 

 zeolite-cavities predominate ; while in the country farther eastward, 

 and especially in the floor of the canon in its lower reaches, quartz and 

 chalcedony prevail as the infilling material. I sought to use this 

 difference as an indication of the position of the basalts in the 

 series, my impression being that the rarity or absence of zeolites 

 marked the lower beds ; but, on descending the plateau as we 

 approached Makwa, and also in the Deka valley, zeolites again 

 became plentiful, although I judged that we there reached the lowest 

 part of the series that came under observation. It is, however, 



1 'The Sedimentary Deposits of Southern Rhodesia' Quart. Journ. Geol. 

 Soc. vol. lix (1903) pp. 267-70. 



2 'The Geology of Southern Rhodesia ' Special Report No. 2, Rhodesia 

 Museum, Bulawayo, 1904, pp. 15-18. 



