144 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Downstream the form of the channel is that of a depressed v-shaped 

 valley, and this finally merges into the broad, open, but flat-bottomed 

 valley, with the high banks far apart. 



The Amazon is also too powerful a stream to be shut up by its own 

 silts. 



Evidence of the islands. — The process by which islands are formed 

 by depression consists in the lowering of the general land surface until 

 the sea passes up some of the valleys and gets behind or landward of 

 some of the highest points. The island of Itamaraca, for instance, before 

 it became an island, had one stream flowing past its northern end — the 

 Tijucapopo (or Catuama) — and one flowing past its south end — the 

 Iguassii — and the canal that now separates it from the mainland was a 

 valley through which small streams drained both north and south. 

 "When the depression of the coast took place the water backed up into 

 the lower parts of these streams and into the valley behind it, thus 

 entirely cutting it oft" from the mainland and leaving it an island. 



The island of Itaparica and the other islands in the Bay of Bahia are 

 also the higher portions of irregular land surfaces let down by a depres- 

 sion till the water of the sea flowed round them. Itaparica channel 

 is a drowned valley that formerly drained northward into what is now 

 the Bahia de Todos os Santos, but which was formerly a valley. The 

 same thing is true of the islands of Tinhare and Boipeba on the coast 

 south of Bahia. 



If we wish to test the theory of these islands having been made as 

 here suggested we have but to imagine the efiect of an elevation upon 

 them. The deepest water recorded in the channel west of Itamaraca is 

 less than three fathoms. If the land were elevated more than three 

 fathoms evidently tliis water would run out and we should have a small 

 stream flowing where tide water now entei's. Many islands on a coast 

 are usually regarded as evidence of a late depression of that coast. It 

 might be supposed, then, that inasmuch as the Brazilian coast has re- 

 markably few islands, this absence of them might be taken as evidence 

 against a late depression. The islands that do exist, however, bear out 

 the theory of a depression, while the absence of a large number of small 

 outstanding islands bears out the idea that the depression took place 

 long enough ago to allow the sea to obliterate -them, and in some in- 

 stances to throw their remains on the shores of the mainland. These 

 same silts also helped join to the mainland some of the in-shore islands. 



Off-shore days. — The borings made by Sir John Hawkshaw on and be- 

 hind the reef at Pernambuco in 1874 penetrated to a maximum depth of 



