Yol. 6s.~] geology oe the Zambezi basin. 207 



To Sir Lewis Michell and to the officers of the British South 

 Africa Company, both in Rhodesia and in London, my thanks are 

 due for the liberal co-operation by which alone it was made possible 

 for me to carry out the field-work in this wild region. 



To Mr. F. "W. Sykes, until recently the District Commissioner 

 and Conservator at the Victoria Falls, who organized and led our 

 expedition on the northern side of the Gorge, I owe more than can 

 be expressed in this simple acknowledgment. 



It is impossible within the allotted limits of space to particularize 

 at length the assistance that I received on all sides in Rhodesia ; to 

 some of which I have, however, been able to make reference in the 

 text of this paper. But my appreciation of the kindness shown to 

 me is none the less keen if unexpressed. 



I have to thank Mr. F. W. Sykes, and Miss Louisa Rhodes (acting 

 as the representative of her brother, the late Col. F. "W. Rhodes, who 

 accompanied us on our journey north of the Gorge), for the 

 photographs with which this paper is illustrated. 



The collection of rock-specimens made during the investigation 

 has been handed over to the Petrographical Department of the 

 Geological Survey of Great Britain, for preservation at the office in 

 tfermyn Street, London. 



Appendix I. 

 Petkographical Notes, by H. H. Thomas. 



The Dolerites. 



Intrusive ophitic olivine-dolerites. 



F 1032 & 1035. Localities: 10 yards from fault at Jack's (Mtoro's) 

 Kraal, Deka Valley ; and 30 yards from fault, Eondulu, Wankie. 



Of the specimens examined, two rocks seem to possess undoubted 

 intrusive characters ; in the hand-specimen they present a fairly 

 compact appearance, without any conspicuous porphyritic mineral. 



The fresh rock is greenish-brown in colour, with small dark- 

 green patches; and F 1082 showed a few minute amygdaloidal 

 cavities, filled with zeolites, chlorite, and secondary quartz. 



Under the microscope these rocks prove to be almost holocrys- 

 talline ophitic olivine-dolerites, of somewhat unusual type. They 

 consist of magnetite, plagioclase with a little orthoclase, and augite. 

 The magnetite is for the most part the earliest product of con- 

 solidation and builds well-formed octahedra, but also occurs in 

 strings and patches. The olivine exists as small idiomorphic crystals, 

 slightly elongated, parallel to the edge (100) (010), and as rounded 

 grains. No fresh olivine now remains, but that it was a variety 

 fairly rich in iron is indicated by the border of iron-ores fringing 

 the pseudomorph. The pseudomorphs are composed of a fibrous 

 highly-refracting mineral, with straight extinction and a yellow 

 to yellow-brown colour, recalling the ' Potluck ' pseudomorphs of the 

 Derbyshire ' toadstones ' or those of some of the Tertiary dolerite- 

 sills of Skye. 



