Dr M. G. A udley -C harles said that the author had argued that western Borneo, which was south of the north-west Borneo geosyncline, was not a 'foreland' but acted as a 'eugeanticlinal ridge and intermediate hinterland'. He would like to suggest that the history and structure of western and southern Borneo 1 could not be determined by consideration of the north-west Borneo geosyncline alone. The south-eastern flank of that 'foreland' or 'eugeanticlinal ridge' should also be considered. The Tertiary geosynclines in central and southern Sumatra, in northern Java and in eastern Borneo 1 (now eastern Kalimantan) occurred on the southern and eastern flank of this 'foreland' or 'eugeanticlinal ridge'. Bemmelen [1949, Geology of Indonesia, The Hague (Govt Printing Office)] described these geosynclines, and their history was briefly summarized by Umbgrove (1938, Bull. Am. Ass. Petrol. Geol. 22 , 1–70). The 'foreland' or 'eugeanticlinal ridge', which separated these geosynclinal basins on the south and east from the north-west Borneo geosyncline, was about 1000 miles long and about 400 miles wide. If the distribution of geosynclines and major structural units in the East Indies were considered as a whole, then the position and function of this 'foreland' or 'eugeanticlinal ridge' in central Borneo appeared to be a very important feature. As Fig. 1 showed, the Indonesian island arcs were displayed in a broad U-shaped loop about 2000 miles wide. In the middle of this loop was the island of Borneo. On the basis of its geological history and its present structure and composition this 'foreland' or