194 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
| » Changes in the in-shore ocean currents, such as are constantly occurring, 
| would remove the consolidated sands from some parts of the reef and 
heap them in other places. 
Some of the conditions here hypothecated exist at present on the 
| * southern coast of California. At and about Oceanside, San Diego | 
d County, the region has been depressed and silts have filled up the stream \ 
| mouths and embayments. The streams are weak and spasmodic, and 
ia unable to keep their mouths open, even when opened by floods. The 
drainage waters collect in pools and marshes behind the long sand beaches, 
and escape by percolating through the beach sands. If these sands are 
F calcareous, we may expect in time to see them hardened and turned into 
| long lime-cemented sandstone reefs very like those of northeastern 
4 Brazil. 
d This hypothesis appears to fit most of the conditions observed along 
| the stone reefs of Brazil: the streams and embayments across which the 
1 reefs Пе are streams and areas of weak drainage ; the reefs have the forms 
and structure of beaches; unconsolidated sands underlie the hard reef 
cap-rock. 
At some places, however, the stone reefs rest against rocks through 
and from which these lithifying waters could not have passed. At Cape 
Santo Agostinho, for example, the northern end of the rock reef laps 
back against granites for a long way north of the embayment behind the 
Barra do Suäpe. The percolation of acid waters from marshes and ponds 
| landward of the present reef might have hardened shore sands in front 
E of these marshes into a rock reef, but the sands that lay against the east 
| face of the cape had no pools to the landward. If they were hardened 
by this process, the acid waters of the marshes must have saturated the 
sands for considerable distance up and down the coast, as well as in 
. front of the marshes. 
The hardening process is not a continuous one. — И the process by 
which the reef rocks are hardened were a continuous one, we might ex- 
pect to find an overlapping series of sandstones filling the embayments. 
This overlapping series does not exist. There are sometimes two or | 
three reefs, one behind the other, but even in these cases the reefs are 
separated from each other by softer layers of sands. The sections of the 
Pernambuco reef obtained by Sir John Hawkshaw also show that there 
are loose sands interbedded with the hard layers. The undermining of 
the reefs by the surf at many places along the coast shows that this 
alternation of hard and soft beds is a characteristic feature of the large 
stone reefs. | 
