The geology of southern Guyana

The geology of southern GuyanaSIR -As a former Director of the Geological Survey of Guyana (1957-62) it gives me great pleasure to welcome a memoir by Dr J. P. Berrange (1977) which describes the geology of the rather inaccessible region of southern Guyana with a most up-to-date professional competence, thus completing in terms of reconnaissance geology a survey of Guyana begun in 1868 by two government geologists and continued over the years, in spite of the logistic difficulties inherent in mapping in a country largely covered with Amazonian rainforest.The detailed descriptions of the Precambrian of southern Guyana given by Dr Berrange are of great value and constitute a notable contribution to the study of the petrological development of Precambrian basement rocks.There are, however, two important points on which Dr Berrange disagrees with previous interpretations by the Geological Survey, and I would like to state briefly why I question his following conclusions:(1) that a laterite, the Nappi Laterite Formation in the interior of Guyana, can be correlated with the high-grade commercial bauxites of the coastal area; and(2) that the important Kanuku Complex of predominantly granulitic rocks covering an area of more than 22000 km 2 should be grouped in the ~ 2000 Ma Trans-Amazonian cycle rather than in the > 2500 Ma Archaean as previously accepted.To clarify my objections to both these conclusions I attempt, with the help of a location map (Fig. 1), to present the facts and principles involved and the reasons for which I think Dr Berrange's interpretations are misleading.