260 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
erosion has vigorously encroached upon the land 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 (1) 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 which 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 
