260 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



erosion has vigorously encroached upon the laud since the elevation of 

 the Tertiary beds, but it has always left a shallow shelf offshore. Then 

 followed a depression which carried beneath the sea some of the valleys 

 near the coast. Many low coastal islands were probably formed by this 

 depression, but most of these islands have long since been removed by 

 marine erosion. In this way it is believed that the Abrolhos islands 

 were separated from the Brazilian mainland. Those islands are low, and 

 the same Tertiary (T) sediments that form them lap back over the crys- 

 talline rocks of the mainland to an elevation of one hundred and fifty 

 metres. The distribution of the Tertiary rocks along the coast of Brazil 

 leads to the supposition that the valley that lay between the present 

 Abrolhos islands and the mainland was a broad shallow one. I say 

 shallow because all the valleys known in the coast Tertiary are shallow, 

 — less than one hundred metres deep. 



The conclusion seems warranted, therefore, that any coral reefs that 

 may have taken possession of this submerged valley since its depression 

 are necessarily of limited thickness, probably not exceeding one hundred 

 or two hundred metres. 



Since Pliocene times there has been a slight elevation of the land, but 

 the coral reefs do not seem to have been near enough to the surface at 

 the time of the elevation to have been lifted altogether out of the water 

 by it. At least no reefs are now known along the Brazilian coast stand- 

 ing quite out of the reach of tide-water. There are several places at 

 which there are dead coral reefs, but for aught that is now known they 

 are dead only because they have reached the upward limits of coral 

 growth. There are also some dead corals to be found among the debris 

 of the elevated beaches of Bahia, but thus far no solid coral reefs have 

 been found among these shell heaps. 



The coral rock that rises above the reef and above tide-water at 

 Maceio is not the remains of an elevated and eroded reef, but the up- 

 lifted corner of a piece of reef rock that has been undermined by the 

 tidal currents. 



In this particular case the thickness of the coral reef appears to be 

 exhibited by the edge of the up-tipped block. It seems to show that 

 the coral reef at this particular place is very thin and rests upon a base 

 of soft material that the marine currents have been able to excavate. 



The profile of the coast in the vicinity of many of the coral reefs also 

 suggests that the reefs must be quite thin. Reference is here made to 

 those places in whicli the reefs lie near a coast having steep bluffs facing 

 the sea — such as exist along the greater part of the coast. The profile at 



