124 bulletin: museum of compakative zoology. 



somewhat greater than the depth of the water between the mainland 

 and the island of Santo Aleixo, that island would of course be joined 

 to the land by this process of elevation. 



But islands are not joined to the mainland solely by this process of 

 elevation ; they may be united by a process of silting up, by the build- 

 ing out of spits or accumulations over an intervening shallow bottom. 

 A convenient illustration of what I mean is furnished by the Pao 

 d'Assucar at Kio de Janeiro. The Pao d'Assucar was not long ago an 

 island having a shallow water passage between it and the Morro de 

 Babylonia just where the Escola Militar now stands. The waves, how- 

 ever, have thrown the coast sands back into this passage until it has 

 been completely filled up and the Pao d'Assucar has thus been joined 

 to the mainland. 



It will be shown later that there has been a depression of the north- 

 east coast of Brazil. Before that depression took place thei'e was a 

 valley southwest of Cabo Santo Agostinho opening broadly toward the 

 sea. In this valley were a few isolated hills left by the erosion of the 

 Tertiary highlands. When the depression occurred, the bottom of this 

 valley went beneath the water, and these isolated hills were left as islands 

 in the big bay. In the course of time this entire bay has been filled 

 with sands and silts and turned into dry land, and in the process these 

 former islands have been united to the mainland. 



At Ilheos, State of Bahia, a rocky hill at the mouth of the harbor, 

 known as Morro de Pernambuco, is joined to the mainland by a sandy 

 neck that has the appearance of being a spit lately built in. It is evi- 

 dent therefore that while islands have been joined to the land, this 

 joining cannot alone be accepted as evidence of an elevation of the 

 coast, but may indeed come from a movement in quite the opposite 

 direction. 



The chief objection to the statement by Capanema as it stands, how- 

 ever, lies in the absence of specified cases. 



7. Tlie straightening of the coast-line (Capanema). — The line of 

 reasoning in the case of the straightening of the coast-line is not indi- 

 cated by Ban\o de Capanema, but it is probably meant that on account 

 of the sea bottom being less rugged and less scored than the land sur- 

 face, an elevation would at once yield a comparatively even beach-line, 

 while a depression, owing to the unevenness of the land, would give a 

 very crooked one. But a coast-line may be straightened by quite 

 another process ; namely, the process of destructive attack by the sea. 

 The facts presented hereafter appear to warrant the belief that the 



