Mtscellaneons. 339 



On Coral-lieefs of the East-African Coast. 

 By Dr. A. Oktmann, of iStrassburg. 



Since the implication of a more detailed treatise upon the subject 

 of my investijjjj'.tioiis into the coral-reefs of the German Kast-Afriean 

 coast will still require some time, 1 ve7iture to communicate herewith 

 a l)rief account of the most important of the results which have been 

 gained. 



The entire East-African coast-region, so far as I explored it, from 

 Zanzibar southward to Mikindani, is one of negative shore-displace- 

 ment. I was able to collect proofs of this at the most widely 

 different spots ; just as, moreover, similar observations are already 

 available for two localities (Zanzibar and Souga-Songa Isl.). It ia 

 probable that the same movement extends to the greater portion 

 of the East Coast of Africa. 



The develojiment of the coral-reefs also corresponds to this 

 negative movement : they accompany the coast throughout and are 

 true shore reefs ('' Strandiiffe "). Their horizontal extension in the 

 direction at right anijlcs to the coast is in close connexion with the 

 slope of the sea-bottom from the shore-line to deep water. Where 

 great depths are found close to the shore (which occurs in our 

 territory chiefly in the south, near Lindi and ilikindani) there is 

 only a narrow shore reef; but where the sea remains shallow to a 

 greater distance from the coast (e. g. in the Mafia and Zanzibar 

 Channels) not only does the shore reef attain a greater breadth, but 

 also isolated reefs are found further outside. I term the latter 

 SHALLOW-WATER REEFS (" Elachsecriffe "). (J. Waltherhas adopted 

 the name pelagic reefs for similar formations in the northern part 

 of the Red Sea : cf. J. Walther, " Die Koralleuriffe der Sinaihalb- 

 insel," Al)h, K. sachs. Ges. Wiss. 2-4 Bd., 18SS.) 



I was nowhere able to observe a formation of barrier-reefs or 

 atolls, and after a careful study of the English Admiralty charts 

 their occurrence appeared to me to be improbable, even at spots 

 which 1 did not visit. As tetje barrier-reefs and true atolls I regard, 

 be it well understood, only those which respectively exhibit a 

 channel or lagoon of great depth and rise from very deep water. 1 

 am firmly convinced that formations of this kind can only arise in a 

 region of positive shore-displacement, and that those cases are of 

 rare and unusual occurrence in which they appear in stationary 

 regions. In this respect, therefore, I abide by the old theory of 

 Darwin and Dana, in opposition to the views recently published by 

 Guppy, who would deduce the existence of negative shore-displace- 

 ments from the actual presence of atolls. The very absence of such 

 reef-formations in our territory is an indirect proof that in regions 

 with negative shore-displacement atolls and the like are not formed. 

 I regard the atolls of the Straits of Jubai in the Red Sea, which are 

 figured by J. Walther Qoc. c'd.) not as teve atolls, in the sense given 

 above, but as atoll-like formations, resulting from the peculiar peri- 

 pheral growth of the corals, which can be observed on a small and 

 large scale in every coral-reef. Moreover the difference between 



