Dr. L. Leigh, Fermor — What is Laterite ? 509 



Xos. 11 to 13, which contain 47 -27 to 56"01 per cent of Fe 3 O g , are not 

 entitled to the terra bauxite, but should merely be termed laterite 

 without any qualification, or, if so desired, ferruginous laterite. 



Samples 14 to 23, comprising Group IV, show 35 - 70 to 6-67 percent 

 of ALOn, 8-77 to 47-39 per cent of Ee 2 8 , 7-96 to 23-32 per cent of 

 combined silica, and 4-53 to 3953 per cent of free quartz. The 

 authors re-arrange these analyses in terms of the mineralogical 

 composition of the rocks they represent; the samples are thus shown 

 to contain 39-94 to 64*56 per cent of my " lateritic constituents " 

 (termed by the authors "Balance identical with Bauxite"), whilst 

 the balance of 35-44 to 60-06 per cent of non-lateritic constituents 

 is made up of 4 - 53 to 3953 per cent of quartz, and 50-26 to 

 17 - 16 per cent of "Clay as Kaolin", with occasional extremely small 

 amounts of Ca 0. The average composition of these ten " detrital 

 laterites " works out as — 



Lateritic constituents . . 49 "85 

 Litliomarge . . . . 30'11 

 Quartz 19-99 



99-95 

 CaO 0-05 



100-00 



or approximately : lateritic constituents 50 per cent, lithomarge 

 30 per cent, and quartz 20 per cent. They are, therefore, on the 

 average just at the upper limit for laterite, and might be termed 

 comprehensively siliceous laterites. 



Taking these ten samples individually, however, differences are 

 recognizable. They may be divided into two groups of five each. 

 In the first group (Nos. 15, 16, 17, 18, and 20) there is a much 

 greater proportion of lithomarge (29 to 50 per cent) than of quartz 

 (4 to 17 per cent), whilst in the second group the quartz (24 to 

 40 per cent) is either approximately equal to or else is greater than 

 the lithomarge (17 to 30 per cent). Of the former division, Ho. 17 

 from Jabalpur (? district) contains 50'26 per cent of lithomarge, and 

 should therefore be termed lateritic lithomarge. It is interesting to 

 note here that the Ponri Hill lateritic lithomarges referred to later 

 also come from the Jabalpur district. The four remaining samples — 

 from belgaura (two), Dharwar, and Birbhum — show over 50 per cent 

 of lateritic constituents, and should be termed lithomargic laterites. 

 These five rocks, with the possible exception of the Dharwar sample 

 containing 17 per cent of quartz, are not, I suspect, of detrital origin, 

 as stated, but were probably formed in situ under conditions in which 

 the lateritizing processes were not pushed to a finish. 



The other five samples (Nos. 14, 19, 21, 22, and 23) all show 

 over 50 per cent and under 60 per cent of non-lateritic constituents 

 (with the exception of JSTo. 23, which contains only 24 per cent of 

 quartz and 20 per cent of lithomarge, and might be designated 

 siliceotis laterite, or even argillaceous quartzose laterite). Judging 

 from the analyses, they are probably detrital rocks, formed perhaps 

 by the admixture of detritus from lithomargic laterite deposits with 



