350 F. P. Mennell — On R/iodesian Late rite. 



lY. — Notes ox Rhodesian Latekite. 

 By F. P. Mbnnell, F.G.S. 



AIS'UMBER of communications dealing with the origin and com- 

 position of the material commonly known as laterite have 

 appeared in the Geological Magazine during the past few years. 

 That of Dr. Maclaren ' is an especially valuable contribution to the 

 literature of the subject, and I think few workers in the Tropics will 

 be disposed to question the substantial accuracy of his conclusions. 

 I shall not attempt, therefore, to discuss the general problem presented 

 by the widespread occurrence of laterite deposits in tropical regions. 

 The object of the present brief communication is merely to place on 

 record a few observations made in another area. 



Condiimis of Formation. — Mr. J. R. Kilroe in his interesting com- 

 munication regarding the German bauxites seems inclined to question 

 the necessity of tropical conditions for the accumulation of laterite. 

 He states - that on this supposition laterite should be met with every- 

 where throughout the Tropics. As a matter of fact, it is. In 

 Rhodesia, for instance, it is by far the most abundant superficial 

 deposit. It may be pointed out, however, that it is not tropical heat 

 which is the essential condition, but the alternation of perfectly 

 distinct wet and dry seasons, which is only observed in or near the 

 Tropics. In Bulawayo (20° S. lat.), for example, we have in an 

 ordinary season about 25 inches of rain, which falls in the summer 

 months, say from about the middle of October to the middle of April. 



'i-"i-i- Decomposed Do. '-V-^ Soft Latepiite oio Hard Uterite. 



Sketch to illustrate a common mode of occurrence of Laterite. 



There may be occasional showers in September or May, but as a 

 general rule June, July, and August are absolutely rainless. Usually, 

 indeed, there is no rain whatever between the end of April and the 

 end of September. Over most of Rhodesia the rainfall is heavier than 

 near Bulawayo, but precisely similar conditions prevail. Scattered 

 over the country, therefore, we find considerable tracts (chiefly on the 

 granite) which are distinctly marshy during the rainy season, but dry 

 up completely when the wet weather comes to a close. Thus among 

 the Matopo Hills there are a number of valleys which may usually be 



1 Geol. Mag., 1906, pp. 536-47. - Op. cit., 1908, p. 539. 



