10 J. C. BRANNER — STONE REEFS ON COAST OF BRAZIL 



sponges, seaweeds, mollusks, and the many other forms of life so abun- 

 dant in tropical seas. These plants and animals form a protective coat- 

 ing that successfully resists the most violent waves. It is true that sea 

 urchins occasionally bore into the reef rock, but the wearing done in this 

 way is insignificant when compared with the protection afforded by 

 other animals and plants. 



Uniform Width of the Reefs 



The nearly uniform width of a reef appears to be due to the ebb and 

 the flow of the tides and the approximate uniformity of the distance the 

 seepage water can move through the sands between tides. The ebb and 

 the flow of the tides are repeated in the sand barriers in so far as the 

 sand will permit ebb and flow. Some reefs are wider than others ; this 

 is probably due, in part at least, to the fact that a larger estuary has to 

 discharge more water in a given time than a small one. There is there- 

 fore more head and more hydrostatic pressure within the sands, so that 

 the water flows farther in a given time when the estuary is larger. As 

 waters flowing from the larger estuaries must movefaster,channels having 

 the same width must have a greater depth, and it follows from what has 

 gone before that the hard strata of the reefs in front of the larger bodies 

 will also be thicker. 



Cause of the Straightness of the Reefs 



The straightness of the reef has never been satisfactorily explained, 

 for there are few beaches or spits on this part of the Brazilian coast as 

 long and straight as the reefs. This straightness is thought to be due to 

 their having been solidified in the early part of their history as spits in 

 front of estuaries, at the time when the spits were narrow and approxi- 

 mately uniform in width. It is believed, however, that some of the reefs 

 are more nearly straight than were the beaches and spits from which 

 they were found. This will appear if we imagine a long beach with a 

 few gentle curves in it and behind it a stream or estuary approximately 

 parallel to the beach, but somewhat more crooked. The waters from 

 both sides of such a spit will meet along a line equidistant from the two 

 water margins, and this line will be more nearly straight than the 

 crooked stream behind or the curved beach in front. If the beach in 

 such a case were straight the reef would be slightly crooked. 



Stone Reef Making still in Progress 



The process of stone reef formation is believed to be still in operation. 

 The new reefs, or perhaps it would be better to say the conditions favor- 

 able to forming new reefs, are now to the landward of the old ones. At 



