28o 



NA TURE 



[August 25, 1923 



the rise in temperature resulting from the blanketing 

 of the sea bottom, by sedimentary deposits, in the 

 neighbourliood of land masses became sufficient 

 considerably to reduce the rigidity of the basic sima 

 beneath the continental shelves. The great thickness 

 of the earliest sedimentary strata suggests that this 

 condition was attained in very remote geological 

 times ; and in view of the slow progress the continents 

 would make by this process of drift, it would appear 

 that the degree of separation now attained by these 

 land masses may be taken to point to a similar 

 conclusion, even though a liberal allowance be made 

 for lateral collapse along the margins of the separated 

 tracts. Accordmg to this view, tensional structures 

 should be dominant throughout the geological history 

 of Africa. 



Of other African territories I will say nothing, but 

 with regard to Uganda, which lies, be it noted, in 

 the heart of the continent and between two great 

 rift valleys, tensional structures are astonishingly 

 absent, or, at any rate, difficult to find. 



Deposited on a basement of crystalline rocks which 

 represents, in all probability, a great accumulation 

 of archaic sub-aqueous deposits intruded upon and 

 largely metamorphosed by ancient acid magmas that 

 have incorporated much of the sediments, is a very 

 thick series of shales and sandstones (usually more 

 or less altered) of great antiquity but of undetermined 

 age. These are part of what we once called the 

 Argillite series (a tentative term now abandoned, see 

 Ann. Rept. Geol. Dept. Uganda, 1920, p. 10) ; they 

 constitute what we now call the Ankolian system. 

 These rocks have suffered much from folding and 

 are sliced up by tremendous faults. Owing to the 

 want of easily recognised horizons within the system, 

 it is usually very difficult to demonstrate the nature 

 of these faults. There can be little doubt, however, 

 that they are essentially compressional structures, 

 and in every instance where the fracture contacts 

 have been seen they have revealed overthrust 

 faults. After this great phase of faulting, the 

 Ankolian beds have been thrown into a series of 

 complicated domes, the eroded remains of which 

 were first described by me as arenas {loc. cit. p. 14). 

 Some of these have been the subject of careful study 

 by Mr. A. D. Combe (Field Geologist, Uganda Service), 

 who has mapped them in detail. It is quite certain 

 that these do not give evidence of tension, but quite 

 the reverse. 



Above the Ankolian, and deposited unconformably 

 upon that system, is the Mityana series, consisting 

 of thick accumulations of sandstones and con- 

 glomerates ; these, too, have suffered from faulting, 

 but to a lesser degree than the Ankolian. The 

 nature of these faults is as yet undetermined. 

 The Ankolian and the Mityana series have together 

 been thrust up by an enormous bathylith (the 

 Mubendi bathylith), the denudation of which has 

 exposed the newer granite : this does not look like 

 tension. Deposits revealing plant-impressions, pos- 

 sibly of Jurassic age, which appear to be the next 

 in order of sequence, have been located in eastern 

 Uganda ; they occur in a synchne of no great size : 

 the significance of this structure is uncertain. No 

 other tectonic movements are as yet known in this 

 country until we come to (probably) late Cretaceous 

 and Tertiary times, when we have the doming of 

 Uganda (the Uganda-Congo dome lying to the west 

 of the syncline of Lake Victoria, which itself lies to 

 the west of the Kenyan dome or anticline). This 

 structure can scarcely be interpreted as tensional ; 

 yet at the time of its inception continental drift, if 

 drift there has been, should surely have been well 

 advanced. The first structures of more than purely 



NO. 2808, VOL I 12] 



local significance that have been interpreted as 

 tensional do not make their appearance until about 

 middle tertiary times, though the action which they 

 signify continued until much later : 1 mean, of course, 

 the nft valleys, and even these, at any rate so far 

 as their first inception is concerned, are more easily 

 accounted for by compres-sion than by its opposite. 



Here, with the Semliki (Semaliki the natives call it) 

 and the Congo rift-scarp to my left, the Toro-Bunyoro 

 escarpment to my right, and the Kuwenzori range 

 behind me, I write sitting on the evidence, as it were, _ 

 that proves, perhaps for the first time conclusively, M 

 the tectonic origin of the Albertine depression, and ■ 

 demonstrates beyond all doubt the amazing fact 

 that early man knew the lake when it stood more 

 than 1000 feet higher than it does now. A thousand - 

 foot head on Lake Albert is impossible to-day, and 

 has been ever since the differential drop of the 

 Bunyoro scarp not only released the pent-up waters 

 of Lake Albert, but gave birth to the Victoria Nile 

 that connects, through Lake Chioga, the great 

 Nyanza with the Albertine depression. All this is, 

 in my opinion, more easily accounted for as a 

 necessary consequence of compressional activity than 

 as the direct result of tension. 



The tectonics of the rift is too big a question to 

 discuss in a letter, but it may be noted that all the 

 evidence that I have been able recently to obtain 

 in Toro and in the Bwamba country supports the 

 view, generally held, that Ruwenzori is an upthrust 

 mass. It is directly connected with the rift but very 

 probably pre-rift in age. Now there is evidence to 

 show that since the inception of the Albertine rift 

 the bottom of the valley has sunk by two distinct 

 major movements well separated in time. The 

 sinkage has been pivotal with a maximum downthrow 

 to the north-east, as has the subsidence of Bunyoro. 

 That which has remained firm and helped in marked 

 degree to hold the sinking bottom of the rift valley 

 is the great faulted upthrust of the Ruwenzori range : 

 this does not look like tension anyway. 



I am afraid that exception must be taken to 

 Dr. Evans's use of the term rift as applied to the 

 separation tract between drifting continents. Thus 

 used, the term is most applicable, but it has priority 

 in Prof. Gregory's usage, which, though it may be 

 less apt, is now unalterable, E. J. Wayland. 



The Semliki Plain, 

 May I. 



Protozoa and Virus Diseases of Plants. 



Attempts to discover the presence of a foreign 

 organism in such diseases as tobacco-mosaic, tomato- 

 mosaic, leaf-roll of potato, and numerous other similar 

 infectious diseases have been the conjcem of botanists 

 for many years. Although considerable knowledge 

 has been gained as to the distribution of these diseases 

 by insects such as Aphides, yet no causal organism 

 has been observed with certainty, and the diseases 

 have been classed accordingly as virus diseases. The 

 failure to detect the presence of a foreign organism 

 has naturally been a serious handicap in combating 

 these diseases, many of which are of serious economic 

 importance. 



The appearance of a paper by R. Nelson entitled 

 " The Occurrence of Protozoa in Plants affected with 

 Mosaic and Related Diseases " (Agric. Expt. Station, 

 Michigan, Bull. 58, 1922) is thus of great interest. 



In this paper Nelson claims that protozoa are to 

 be found in the phloem of plants affected by bean- 

 mosaic and tomato-mosaic, and also in potato plants 

 affected by leaf-roll, while such organisms are absent 

 from the phloem of healthy plants. 



