Dr. L. Le'ujh Fermor — What is Laterite ? 559 



The only other area where ' Treniadoc ' rocks occur in the 

 St. David's district is at Tremanhire, north-east of Solva. They have 

 been examined recently by Mr. H. H. Thomas and Professor 0. T. 

 Jones, who kindly inform me that they consider the rocks to be un- 

 doubtedly of Arenig age. They yield an Ogygia, probably 0. sehvyni. 



V. — What is Laterite ? 



By L. Leigh Fermor, A.E.S.M., D.Sc, F.G.S., Geological Survey of India. 



{Concluded from the November Number, p. 516.) 



Laterite a stratigraphical as ivell as a petrographical term,. — In this 

 paper I have urged the advisability of applying the terms laterite, 

 lithomargic and quartzose laterite, lateritic lithomarge, etc., to certain 

 rocks, all of which some geologists would include under the term 

 laterite. This restriction of the use of the word laterite is advocated 

 in the petrographical sense. Besides being used as a rock name, this 

 word is also applied as the name of a geological formation in India, 

 ranging from Tertiary, through Pleistocene, into recent times, 1 for 

 probably in some parts of India, as in other parts of the tropics, 

 laterite is still in process of formation. As the name of a formation, 

 the term laterite comprises, not only substances that petrographically 

 are true laterite, but also lithomargic and quartzose laterites, lateritic 

 lithomarges, and lithomarges that are practically free from lateritic 

 constituents, not to mention various laterite-cemented detrital rocks. 

 Consequently, in mapping geologically in India it is necessary to 

 include as laterite many rocks that petrographically are not laterite. 



The parallel to such a twofold use of the word is easy to find. The 

 Charnockite Series includes members that are not charnockite. The 

 Bengal Gneiss is the name of a geological formation containing not 

 only gneisses, but also limestones, dolomites, and mica-schists. No 

 one wishes to call a piece of limestone from this formation " a piece 

 of Bengal gneiss ". Similar examples occur in the stratigraphical 

 nomenclature of most countries. 



It is probably failure to notice this twofold application of Buchanan's 

 original term in Indian geology that has led geologists in other parts 

 of the world to a comprehensive use of the term petrographically, as 

 well as strati graphically. There can be no objection to engineers 

 using the term in a wide sense, for although some of the substances 

 designated laterite by them will not be laterites in the petrographic 

 sense, yet they will in most cases belong to lateritic formations. 



IV. The Application of the term Bauxite. 



The question whether the word bauxite should be retained as the 

 name of a definite mineral or not is not difficult to answer if one 

 appeals to the historical side of the subject. P. Berthier, 2 in 1821, 

 described a supposed iron-ore {mineral de fer) from Beaux near Aries 

 in Provence, which he had analysed and found to be " compose 



1 Manual of the Geology of India, 1893, p. 369. On p. 385 the use of this 

 word as a lithological and a chronological term is discussed, and the former use 

 preferred. 



2 Ann. des Mines, vi, pp. 531-4. 



