Geology. — ''On Reef caps" . By Prof. H. A. Brouweh. (Commu- 

 nicated b}^ Prof. G. A. V. Molengraaff). 



(Communicated in the meeting of November 30, 1918). 



Proof of an n[)lift of the land relatively to the level of the sea 

 can, in tropical regions, often be supplied by the presence of up- 

 heaved fringing reefs. Thus it will be seen that in the eastern part 

 of the Indian Archipelago, where an elevation of the row of islands 

 has taken place on a lai-ge scale ever since the plio-pleistocene 

 period, upheaved fringing reefs, often forming continuous reefcaps 

 occiii- in most of the islands, sometimes to a height of about 1300 m. 



The character of the latest movements in the curving row of 

 islands of the south-eastern archipelago, which resulted in') the 

 formation of these islands, has been discussed in an earlier paper '). 



The latest mouulain building is considered to be a revival of the 

 intensive young-tertiary movements, the typical features of the islands 

 indicating that, just as in the case of the tertiary, also with the 

 youngest mountain building the movements are in the direction of the 

 "Vorland" whereas near the surface mostly faulting is observed. The up- 

 heaval of the islands has not been simultaneous, nor equally intense 

 in all places, while periods of temporary subsidence have probably 

 interrupted the general elevation since the plio-pleistocene period. 

 In this paper we shall try to ascertain whether there is any relation 

 between the widely varying characters of the leefcaps we observed, 

 and the character of the crustal movements. In this endeavour the 

 aspect of the movement of the geanticlines during long periods will 

 be brought to the front more than has been done heretofore. 



The reefs at the time of their growth. 



The growth of fringing reefs along parts of the coastline may 

 be prevented by various causes, e.g. the lack of a solid substratum, 



1) G. A. F. Molengraaff. On recent crustal movements in the island of Timor 

 and their bearing on the geological history of the East-Indian Archipelago. Proc. 

 Kon. Ak. V. Wet. June 29, 1912. 



') H. A. Brouwer. Over de bergvormende bewegingen in het gebied der boog- 

 vormige eilandenreeksen van het oostelijk gedeelte van den O. I. Archipel. Vers). 

 Kon. Ak. V. Wet. XXV. p. 768. 



