818 



other factors besides time, wiiicli govern the influence of erosion 

 on the reefcap, e.g. the nature of the substratum on which the 

 reefs have been deposited and the power of resistance of the 

 reefs themselves. If the substratum consists of soft I'ocks, which 

 bring about landslips, while deep valleys are cut in the formations, 

 as is the case with a great part of the mesozoic deposits in a number 

 of islands of the eastern Indian aivhipelago, the uplifted reef over- 

 lying it will soon ciumble away. 



If the reef is merely a thin crust covering the undei-ground, it 

 will disappear the sooner; thick reefs will resist erosion for a con- 

 siderable time, and will occasionally act as a protective cover over 

 a soft underground. 



As already observed, thick reefs will form in places, where the 

 coastline maintains itself for a considerable time, or has undergone 

 only more or less horizontal or downwai-d movements. As such they 

 will afterwards constitute parts of the reefcap, whereas in those 

 places where the coastline has long been exposed to strong negative 

 movements only a thin reef can be evolved, which later on will 

 occur as a thin part in the reefcap. This part is liable to disappear 

 through erosion. For it is just with these strong negative movements 

 that erosion often acts very forcibly, so that both factors co-operate 

 to remove the effects of these movements from the reefcap. With 

 short negative movements this will be manifested only in a terraced 

 structure. 



It will, therefore, frequently be seen that, at great heights above 

 the sea-level, the reefcap is fully developed, whereas lower down 

 towards the coasts it has totally disap|)eared or has been preserved 

 only in detached fragments, while on the coasts themselves living- 

 corals are thriving well. Here we are reminded of our investigations 

 in various localities along the north, coast of the island of Rotti, 

 along the coasts of Serinata, Great -Obi, Ceram and Timor, where 

 the lower elevated reefs (if still any have been left) are for the 

 greater part removed by erosion, e.g. if they have been preserved 

 only on the ridges between the valleys, draining towards the coast. 

 Along the North coast of Timor the thick reef of the Talau basin 

 abruptly terminates near Bahbo at a height of ±610 m. ^), between 

 Balibo and the actual coast no trace of elevated coral reefs is found, 

 whereas at the coast living corals are abundant. Here the reef may 

 have been removed by erosion, in which process the above-mentioned 

 conditions of a rapid erosion must have been present, while the 



') G. A. F. MoLENGRAAFF, loc. cit., p. 128. 



