88 PKOCEEDINGS OF TOE GEOLOGICAL SOCIETY. 



India and Madagascar, I wish briefly to call your attention to the 

 Mozambique Channel. This channel is 250 miles broad at its nar- 

 rowest part and upwards of 1000 fathoms deep throughout ; the 

 least recorded depth (which is close to the African coast in the 

 narrowest part of the channel) being 1130. No one questions for 

 a moment that Madagascar and Africa were united during part of 

 the Tertiary era ; the large mammalian fauna of Madagascar alone 

 amply proves the fact. As already mentioned, only two genera of 

 mammals are common to Madagascar and Africa, though a few 

 species of reptiles and batrachians are found in both. One of the 

 Mammalian genera common to both areas is Crociduy^a, probably an 

 ancient type, but also possibly introduced by man ; the other is 

 Potamoclioenis, a kind of pig. Now no other ungulate, except this 

 pig, is found in Madagascar, and hence it is probable that all 

 the South African Ungnlata belong to the Miocene and Pliocene 

 European fauna, which is believed to have migrated into South 

 Africa after the separation of Madagascar. As Wallace has 

 pointed out, all pigs swim well, and Potamochoerus is said to be 

 more of a water-animal than most pigs, and may very probably have 

 crossed from the mainland after the lemurs, insectivores, and other 

 mammals had been isolated by sea. But how far could Potamo- 

 clioerus swim ? Surely it is not likely that it could cross the Straits 

 of Dover. I think we are justified in assuming about 10 miles as a 

 probable limit of its power of crossing the sea, but, to be safe, let us 

 suppose double as much *. Then, in Pliocene or Pleistocene times, 

 quite as probably the latter as the former, when Potamochoerus 

 reached South Africa, Madagascar was separated by a channel not 

 more than 20 miles broad. The conclusion is inevitable that nearly 

 the whole depression of upwards of 1000 fathoms is of Pliocene or 

 Post-pliocene date. Of course it must not be understood that this 

 date is proved. What we may consider, however, as beyond any 

 doubt is that the depression cannot be older than Middle Tertiary. 



4. Madagascar and India. — The question whether there was in 

 Secondary or Tertiary times land-connexion across the Indian 

 Ocean between India and Madagascar has been treated at con- 

 siderable length, with great ability and literary skill, by Mr. Wal- 

 lace in the ' Geological Distribution of Animals ' and in ' Island 



* Elephants are excellent swimmerg, and have been known to swim, without 

 a rest, for six hours, and, with a rest, for nine. But the pace is A'ery slow, 

 little, if at all, more than a mile an hour. 



