164 BULLETIN : MUSEU:\r OF COMPARATIVE ZOOLOGY. 



These cases all appear to favor the second hypothesis. It is a very 

 noticeable feature of the first hypothesis that it requires still earlier move- 

 ment and time for the formation of the deposits, while the second hypoth- 

 esis satisfactorily accounts for all the deposits with only two movements. 



Taking the case of the Sao Thome valley the sequence is also satis- 

 factorily established. A glance at the region shown in the sketch map 

 on p. 150 shows tliat it has had the following geographical history : 

 Subsequent to the deposition of the Tertiary beds, these rocks were com- 

 pressed more or less, lifted out of the water, and subjected to decora- 

 position and erosion that carved out rather steep-sided valleys. There 

 then followed a depression that admitted the water into the whole 

 region now covered by the Bay of Bahia — and a little more. The 

 npper ends of these valleys began at once to silt up with sands, shells, 

 and the like. After they were already considerably shoaled by this 

 process, there came a slight elevation that brought the shallow ends of 

 these valleys out of the water, and left them exposed on their margins 

 here and there to the action of the waves. 



Thus the second hypothesis of a larger depression followed by a 

 smaller elevation appears to fit all cases, and for the present at least 

 must be considered as the correct one. 



Influence of the coral reefs. — A factor that has been an important 

 one in controlling the outlines of the northeastern Brazilian coast is the 

 coral reefs. Their influence has been both protective and constructive. 

 There are included under this head, not only the corals proper, but 

 likewise the Serpulae and other lime-secreting organisms that live at- 

 tached to the rocks and reefs. Without such protection, the stone reefs 

 themselves would probably long ago have been obliterated. 



The forms of the shores behind the reefs stand in evidence of the 

 great influence of the reefs upon these shores. Wherever there is a 

 reef of considerable length, a sand spit reaches out from the land to meet 

 it. Unfortunately the coast charts are not sufficiently detailed to show 

 this. On the ground it saut au.r yeiu\ 



The small harbors south of Pernambuco nearly all open behind breaks 

 in the reefs. That at Tamandare is an example. North of the bay the 

 point on which Tamandare stands approaches the coral reef north of a 

 break, while Ilhetas Point approaches it in the same way south of the 

 break, leaving the bay a semicircular one. 



Maceio is another example, though in this case the harbor is behind 

 one end of the reef. 



The coral reefs have already caused a general advance of the shores 



