THE PROCESS OF REEF BUILDING 9 



the loose sands a rock barrier against the onslaught of the sea itself. It 

 is now clear why the hard rock occurs only at the top of the reef and 

 why it is not thicker. It is because the seaward percolation of the fresh 

 water is not likely to occur at a depth exceeding or much exceeding the 

 depth of the stream behind the sand spit, and it seems probable for this 

 reason that the hard top stratum varies somewhat in thickness and in 

 width, the stronger streams having the thicker and broader reefs in front 

 of them and the weaker ones having the thinner and narrower reefs. 



Coast Changes 



A coastline is never quite at a standstill. The shore currents average 

 a little different one year from those of another. Coral reefs and the 

 average of the winds during a given season help on some of these 

 changes. The result is that there is always a tendency to cut a little 

 more at one point and to fill a little more at another. In the case of 

 such a spit as has been described these changes may in time either bury 

 it under new accumulations of sand. or its unconsolidated edges may be 

 undermined on either side or on both sides until its edges break down. 

 If the hardening process has not proceeded far enough at the time of 

 encroachment the entire structure may be broken up. It is evident that 

 many reefs have thus been attacked by the sea and partly destroyed 

 before the consolidation was completed. Broken lines of fragments 

 mark the position of many such reefs along the Brazilian coast. Even 

 the strongest of the reefs are undermined here and there, so that the 

 tides ebb and flow beneath them, or natural arches so formed have 

 been broken down, and their angular fragments fill the gaps. Some 

 reefs are buried or partly buried beneath loose sands. At Rio Formoso 

 the reef passes through a sandy cape and sticks out on both sides of it. 

 Several reefs are buried at one end and exposed at the other, while still 

 others are exposed their entire length. 



Protective Agents 



But while some reefs are broken up by the surf many others have 

 resisted for hundreds of years, perhaps for thousands, without showing, 

 during historic times at least, any marked signs of change. When a reef 

 is partly undermined, especially on the seaward side, the fragments that 

 break off fall with their outer margins considerably lower than the inner 

 margins. These pieces, on sinking to the bottom, so lie as to afford pro- 

 tection to the rest of the reef. Waves striking these flat blocks glide up 

 their sloping surfaces and lose their force before hitting the main reef. 



Another important factor in the preservation of the reefs is organic 

 li fe. The seaward faces of the reefs are everywhere covered with serpulae, 



II— Bull. Geoi,. Soc. Am., Vot. 16. 1904 



