,^,. GEOLOGY OF BORNEO. 115 



The diluvium (drift) of the plains occurs chiefly as a zone 

 round the Tertiary hill-land, though it also covers the flanks of the 

 mountains. It is of great practical importance, as it contains the 

 chief deposits of gold, platinum, and diamonds. It may be described 

 as solid flat land in contradistinction to the marsh-, hill-, and mountain- 

 land. Its composition is fairly uniform, its upper part consisting of 

 clays and sandy clays, its lower of pebble beds. It is between the 

 pebbles of the latter that the precious metals and stones are found, 

 mixed with a fine clayey earth. In some places the pebbles are 

 united by a siliceous cement to a hard conglomerate. 



The alluvium of the marshes has a wide distribution in Borneo. 

 Its mode of occurrence has been already referred to. The marsh- 

 land rises very slowly from the coast. This is shown in the lower 

 course of the rivers by the great distance from the mouth at which 

 the influence of the tides is still perceptible. The Barito, for instance, 

 is tidal for fifteen geographical miles, while in certain seasons the 

 influence of the tides may be observed at a distance of thirty-five 

 geographical miles. It is also shown by the immense area covered 

 by the diurnal and periodic floods. The area flooded daily in the 

 basin of the Barito is estimated by Schwaner at 160 square geo- 

 graphical miles, or one-twelfth of the whole river-basin, to which 

 420 square miles must be added in the rainy season. Consequently, 

 during the west monsoon, 580 square geographical miles, or more 

 than one-third of the river basin, are under water. 



With regard to the geological evolution of Borneo, Posewitz's 

 ideas are not altogether in unison with those that have been expressed 

 by Wallace in respect to the whole of the Malay Archipelago. While 

 Wallace regards these islands as having been produced by the break- 

 ing up of a continental area, Posewitz describes Borneo as resulting 

 from the fusion of an archipelago of small islands. The grouping of 

 the different island-clusters in this archipelago has been preserved in 

 the main features of the present structure. A narrow island, which 

 extended from north-east to south-west, is now the Tanah Laut 

 range. Parallel to it was the mountainous island of Pulu Laut. The 

 Pramassan Alai and Amandit mountains were represented by islands 

 of the same general type. To the north there extended a broad sea, 

 lapping round the foot of the central mountains. A few peaks rose 

 from its surface, marking the direction of the south-east mountain 

 chain. In the south-west was a large cluster of small islands, which 

 are now the mountains of Sukadana and South-West Borneo. A 

 large island occupied the position of the present Chinese districts, 

 and from it a chain of islands extended to the great mass of Kina-balu 

 in the north-east point of Borneo. 



In the beginning of the Tertiary period the seas surrounding 

 these islands began to be filled in by the deposition of sedimentary 

 matter. Borneo now acquired a shape resembling that of the neigh- 

 bouring island of Celebes. Great gulfs and arms of the sea, fringed 



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