Correspondence — T. G. Bonney. 431 



a considerable depth ", show worn -down surfaces of low relief. 

 Holmes is quoted as saying that " the steep slopes of the Inselberg^ 

 peaks and mountain blocks [of Mozambique] are always free from 

 deposits of lateritic constituents " ; but the stage of physiographic 

 evolution in areas where laterite occurs is not mentioned. In 

 Fermor's review of the studies by Lacroix on laterites in Guinea, 

 to which reference is made by Mr. Dixey in a footnote, it is said 

 that '' lateritization is everywhere intense where the slope of the 

 ground is low enough to permit the infiltration of water and allow 

 it to remain for a long time in contact with the rocks " (Geol. Mag., 

 1915, p. 128). As laterite is usually developed on deformed or 

 crystalline rocks, the most probable means of giving them a surface 

 of low slope is by long-continued degradation ; that is, by permitting 

 their physiographic evolution to advance to a late stage in a cycle 

 of subaerial erosion. 



The principle here involved has found application in the active 

 search for manganese ores by members of the U.S. Geological Survey 

 during the War ; for the ore was often found to be related to some 

 former lowland of erosion, now uplifted and more or less dissected. 

 The nickel ore of New Caledonia is similarly situated ; it occurs on 

 uplands which, as far as I could judge, during a trip around that long 

 island in 1914, are residuals of an uplifted and submaturely 

 dissected peneplain (see " Metalliferous Laterite in New Caledonia " : 

 Proc. Nat. Acad. Sci., vol. iv, 1918, pp. 275-80). 



W. M. Davis. 



Harvard University. 



May, 1920. 



GLACIAL EROSION. 



Sir, — I must leave it to my friend Professor Gregory to discuss 

 in detail Professor W. M, Davis' paper on " The Glacial Erosion of 

 Snowdon ", but as the question covers a far wider area, I should 

 like to state that I still think glacial erosion to have been, com- 

 paratively speaking, an agent of minor importance in the formation 

 of mountain valleys. My views were expressed in the paper on 

 " Alpine Valleys in relation to Glacier^ " (Quart. Journ. Geol. Soc, 

 1902, p. 690), in which I refer to three others in the same journal 

 in 1871, 1873, and 1874, which three were the fruit of some fifteen 

 years' work. Since then I have visited the Alps (more than thirty 

 three times in all), the Pyrenees, and several other regions, with 

 this question always in mind, with the result that I doubt whether 

 Professor Davis has really explored any of the regions which I 

 describe. How many of the lateral valleys in the Alps has he 

 ascended to their head ? Has he seen the Creux de Champ, 

 the Fer a Cheval, the Am Ende der Welt, the Croda Malcora, 



^ Is the English language really so geographically incompetent that we must 

 say Inselberg, Hinterland, and Thalweg instead of residual mountain, back 

 country, and stream line ? 



