Dr. L. Leigh Fermor — What is Laterite ? 515 



Table. — The conclusions drawn above on the nomenclature of 

 laterites are summarized in the accompanying table, which is self- 

 explanatory. The limits between the various groups have, of course, 

 been arbitrarily fixed, and might be adjusted to suit local convenience, 

 but not to any serious extent. 1 



Lateritoid. — Through manganese-ores and iron-ores, the laterites 

 are related to the rocks I have called lateritoid, shown in the lowest 

 line of the table, and as I consider it rather important to distinguish 

 between those two related rocks I will quote in full my original 

 account of lateritoid l — 



" Of the many occurrences of lateritic manganese-ores that I have been able 

 to examine, only three were in what would be called 'laterite', without any 

 hesitation, by all geologists. These three were in the low-level laterite of Goa ; 

 in the high-level laterite of Talevadi in Belgaum ; and near Gosalpur in the 

 Jabalpur district in laterite that must, I suppose, be called ' high-level ', 

 although it is only at a level of about 1,300 to 1,400 feet. The remainder 

 occur in a rock that some geologists would probably designate ' laterite ' ; but 

 others would probably object to the application of the term. The rock to which 

 I refer has a lateritic aspect and usually consists of a cavernous mixture of 

 various oxides of iron, chiefly hard limonite, yellow ochre, and soft hematite. 

 When no other constituents are present, the rock often resembles typical laterite 

 in its structures and mineral composition so closely that when detached from 

 its rock masses it could not be distinguished from pieces of ordinary ferruginous 

 laterite (of non-detrital origin). Fairly often, however, it contains ores of 

 manganese, either wad, psilomelane, or pyrolusite. The iron-ores and 

 manganese-ores are mixed with one another in a very irregular manner. Veins 

 and patches of the manganese-ores sometimes occur in the iron-ores, whilst in 

 other places veins and patches of iron-ores occur in a mass consisting chiefly of 

 manganese-ores. Owing to the cavernous character of the rock the limonite 

 and psilomelane have often been able to develop marked concretionary structures, 

 such as botryoidal or stalactitic, whilst the pyrolusite is often in small crystalline 

 aggregates. This rock does not always consist entirely of iron and manganese- 

 ores. It often contains patches of quartz, quartzite, slate, or phyllite. 

 Examination with the microscope shows that these rock-patches are residual 

 pieces of rock set in a matrix of ore, and that the latter has evidently been 

 formed by their replacement. Examination of the masses of rock in the field, 

 especially as revealed in the workings for manganese, confirms this deduction, 

 and shows that there is a downward passage from the lateritic mass of iron and 

 manganese-ore at the surface through rock containing more and more quartzite, 

 slate, or other rock, and less and less ore, to a rock that is free from all signs 

 of ore. The junction between the overlying lateritic mass of rock and the rock 

 on which it rests is consequently an extremely irregular one. There is, in my 

 opinion, no doubt that these lateritic masses have been formed by the meta- 

 somatic replacement of the quartzite, slate, or phyllite, accompanied by 

 segregative changes. And as these rocks do not usually contain more than 

 a very small proportion of, and often no, manganese, it is evident that the 

 manganese must have been largely brought in from outside by percolating 

 waters, with the resultant replacement from the surface downwards of the 

 particular rock that happened to be at the surface where the replacement took 

 place. These lateritic replacement deposits nearly always occur as cappings to 

 small hills, the actual rugged surface of the rock (often spotted with lichens) 

 being approximately horizontal. These caps do not, however, show signs that 

 they are the remains of a horizontal sheet, for they occur at different elevations 

 on neighbouring hills, and as noticed above have a very irregular base. And 

 although there are doubtless many cases where such a cap has been cut into 

 two by erosion, yet most of the occurrences suggest that they have been formed 



1 Mem. Geol. Surv. India, xxxvii, pp. 381-3, 1909. 



