430 Correspondence — W. M. Davis. 



which appears to be of jirime importance, namely, a late stage in 

 a cycle of subaerial erosion, when the slow removal of rock waste 

 from a worn-down land surface permits deep penetration of 

 weathering with very slow movement of ground-water, and thus 

 allows time for the various chemical reactions upon which lateritiza- 

 tion so largely depends. The general statement of this aspect of 

 the problem is as follows. 



Conceive the changes suffered by a recently elevated region, 

 composed mostly of such rocks as commonly underlie laterite. 

 In the early or youthful stages of the cycle of erosion thus intro- 

 duced, narrow valleys will be incised by the main rivers ; the 

 steep valley sides will exhibit bare rock or will be thinly covered with 

 coarse rock waste which creeps rapidly down the slope to the river 

 below, while the ground- water percolates downward more rapidly 

 still. Laterite cannot be formed under such conditions. In a later 

 or mature stage of the cycle, many branch valleys will also be incised, 

 and all the valleys will be opened to slopes of a moderate degree ; 

 both strength and variety of relief will thus reach their greatest 

 values. The valley sides will then be covered with a thicker cloak 

 of finer rock waste, but the down-slope creep of the waste and the 

 downward percolation of the ground- water will still be comparatively 

 rapid ; little or no laterite mil be formed. In still later and much 

 longer-enduring stages of the cycle, the valleys having been already 

 deepened about as much as possible, the inter-valley hills will be 

 slowly subdued to moderate or small relief, and their slopes will be 

 reduced to gentler and gentler declivity. Thus a condition is reached 

 in which the removal of rock waste from the general surface of 

 the region and the movement of ground-water are very much 

 slower than they were on the side slopes of the mature valleys. 

 Weathering will now enter to greater and greater depths, ground- 

 water will linger in the deep soil, and chemical changes will have 

 time for deliberate operation. If no upheaval of the region takes 

 place, the subdued hills may eventually be worn down to the gentle 

 undulations of a peneplain, in the old age of the cycle, and on such 

 a surface the processes of lateritization find their optimum. 



If upheaval takes place after the peneplanation and lateritization 

 of a region, the laterite will be quickly removed where narrow 

 valleys are rapidly incised in the early stages of the new cycle of 

 erosion ; but the laterite will remain on the upland areas until, 

 as maturity is approached and reached, these areas are invaded 

 and consumed by branch valleys. The occurrence of laterite in 

 different parts of the world gives much support to this view of its 

 origin. 



The relation of lateritization to physiographic old age thus involved 

 is indirectly implied in Mr. Dixey's statement that " in the more 

 hilly and mountainous parts of [Sierra Leone] . . . laterite is not 

 nearly so well developed " (p. 214). It would be interesting to learn 

 if the other areas, where " the solid rocks have been lateritized to 



