Mr Crossland , The Coral Reefs of Zanzibar. 503 
Another instance of the good effect of moving water upon corals 
is the fact that the only place where corals occur near Zanzibar 
town is in and around certain clear salt-water springs on the 
shore of Mbweni Bay, the rest of the shore being bare mud. 
Darwin 1 quotes similar inexplicable absences of coral reefs from 
large areas, viz. west coast of S. America, the Galapagos islands, 
Gulf of Panama , west coast of Africa and islands of the Gulf 
of Guinea, St Helena, Ascension, Cape Verdes, St Paid's, Fernando 
Noronha. He remarks that too great variation of temperature, 
recent volcanic action, may account for some of these cases, 
but he “ cannot see that,” in the cases printed in italics, “ the 
absence of coral is explicable through any known cause.” 
The islands of Bermuda 2 afford a most interesting comparison 
to Zanzibar. They are composed of aeolian limestone whose 
properties are similar in many ways to those of raised coral, 
thus the forms of rocks exposed to erosion and their pitted surfaces, 
as shewn iu Verniks and Heilprin’s 3 photographs, recall the rocks 
of East Africa very strongly. There is no question of the origin 
of these reefs by erosion, living coral forming only “ an imperfect 
coating” (Verrill) to the aeolian limestone. Erosion has formed 
not only a semblance of fringing reefs, but has gone so far as to 
make what Verrill names a “pseud-atoll.” 
I wish to express my gratitude to Mr Stanley Gardiner, who 
has given much help by discussing and reading this paper. 
1 Coral Reefs, p. 81. 
2 Verrill, Avier. Journ. Sci., Ser. 4, vol. ix. 1900, p. 313. 
3 Heilprin, The Bermudas, The Contemporary Publishing Co., Philadelphia. 
