POSTSCRIPT. xci 



graphy of all the country South of the great river. Something 

 more is, however, required on a question of such interest. 



When was the great chasm formed? There seems to be 

 no better way of gaining an approximate answer to this ques- 

 tion than by learning the nature of the shells which inhabited 

 the central lake when the calcareous tufa was formed ; but on 

 this point we have little information in the Missionary Travels. 



The great chasm does not extend above the falls. What are 

 the rocks in the river-bed above the falls ? How far do the 

 rocks extend towards the great swampy plain down which the 

 Zambesi descends to Kalai? Are there any traces of the cal- 

 careous tufa to be seen on the swelling ground which skirts 

 the river near Kalai? These questions will, we trust, be well 

 examined by the gentlemen of the next expedition up the 

 Zambesi. 



6. Geology. On this subject I shall be very short, and I should 

 be so though time were less pressing than it is. For no geolo- 

 gist will be content with second-hand opinions ; and a reader 

 who knows little of geology would not thank me for dry details 

 on a subject in which he takes no interest. We have some valu- 

 able published details respecting the geology of the Cape and 

 the neighbouring country. Some parts of the country are cer- 

 tainly palaeozoic ; and other parts may be of the old secondary 

 period. As to the great eastern and western coast-chains, 

 we believe that several parts of them are metamorphic and 

 palaeozoic; but of their structural and stratigraphical details 

 we know at present very little. 



As a mere matter of opinion, founded only on lame English 

 analogies, I should expect that, when its fossils are explored, the 

 coal-field near Tete will turn out to be of a true palaeozoic or an 

 old mesozoic period. 



The bearings of the eastern and western chains of South 

 Africa are so nearly North and South, that if there were any 

 true physical foundation for the hypothesis (first advanced by 

 the illustrious Humboldt and afterwards adopted by Sir R. I. 

 Murchison) that such North and South bearings are an indica- 

 tion that the rocks are auriferous — then we ought assuredly to 

 expect auriferous deposits in various portions of these great 



