386 SECTIONAL TRANSACTIONS.— H. 



geological significance during which rainfall was in general considerably heavier than 

 in earlier times, and than it is to-day, over an area sxijficiently large to be of some account 

 in loorld events. 



E^adence of relatively high precipitation in the past is not difficult to obtain. 

 What is required, however, is a body of geological fact that can only be interpreted 

 as arising from an increase of rainfall, and, after long sustainment, a subsequent 

 decrease to that of the present day. 



In the case of high lakes, the high level could be engendered by a decrease in 

 evaporation, but it does not seem probable that rainfall would remain unaltered if 

 any considerable change in evaporation took place. Fortunately, a long-sustained 

 high lake is likely to leave behind it some evidence of its origin (e.g. in Uganda, damming 

 of erosion valleys by lava flov/s, tectonic tilt, depression of ground) ; but one which 

 owes its existence to heavy rainfall will display not only evidence of high levels in 

 the form of wave-cut cliffs and beaches, etc., but also of high level built-up gravel 

 terraces of deltaic origin at the mouths of affluent rivers. 



Pluviation pure and simple is incompetent to produce river-terraces in a valley 

 already at grade — an essential for terrace-production is an alteration of base-level. 

 The problem is, what is the cause of this alteration ? The answer involves sorting 

 out tectonic and climatic factors, so far as they are represented. 



The Uganda evidence in relation to pluvial periods may be classified under five 

 heads : (a) Heavy rainfall ; (b) Aridity ; (c) Change from drier to wetter conditions ; 

 (d) Change from wetter to drier conditions ; (e) Chronological evidence. Only the 

 last-named can be touched upon here. 



Chronological Evidence. — 1. The great African pene-plain has been more than 

 once disturbed. Such disturbances occurred in late Pliocene or Early Pleistocene 

 days, and again, with very marked effects, in post-middle or late-middle Pleistocene 

 times. 



2. In the history of climatic changes, climatic and tectonic effects are intermixed, 

 and the problem has been to disentangle them. The first climatic event with which 

 we are concerned was a long period of precipitation, in all probability in late Pliocene 

 and/or early Pleistocene. 



3. The next climatic event is a dry period in the middle Pleistocene. 



4. Then followed a period of heavy precipitation in middle Pleistocene, or post- 

 middle. Extremely important earth-movements occurred during this period. 



5. A very marked oscillation to arid conditions followed, and was succeeded by a 

 relatively wet climate, apparently not of long duration, and by no means as severe as 

 the two preceding wet periods. 



6. Associated with the wet periods are definite Stone Age cultures. 



The geological evidence leaves no room for doubt that the first two wet periods 

 were of long duration, and there can be hardly any doubt that they were true pluviale. 

 So far as dating goes these appear to correspond each with a pair of recognised glacial 

 periods, that is to say, Pluvial I was more or less contemporaneous with the Gunz 

 and Mindel glaciations, whilst Pluvial II was more or less contemporaneous with the 

 Riss and Wurm glaciations. The two pluvials were separated by a marked dry 

 interpluvial period. 



Recent investigations by Dr. G. C. Simpson into the probable cause of the great 

 Ice Age resulted in a theory which demands precisely those climatic conditions that 

 appear to have obtained in eastern central Africa during the Pleistocene ; and all 

 available evidence favours the idea that these two pluvial periods were true pluvials 

 in that they were contemporaneously manifested over wide areas of the earth's surface. 



Geological investigations, at any rate, must go much further before we can be 

 justified in adopting, with complete confidence, any such correlation as that which 

 I have accepted ; meanwhile the suggestion provides a working hypothesis. 



Afternoon. 

 Excursion to Bath. 



Evening. 



Prof. G. B. Barbour. — The Geological Background of Peking Man 

 (Sinanthropus). 



