Northeast Coast of Brazil. 369 



feet thick at most, and is underlain by the usual coastal sands 

 and clays. The outer edge of a reef is in many places nearly 

 vertical, due to the breaking off of masses which now lie 

 beside it in inclined positions and partly protect the exposed 

 face from the force of the surf. The inner face is commonly 

 broken much less, but its edge is irregularly etched. In 

 many places the surface of the reef is also deeply etched into 

 fantastic points and pinnacles, and contains shallow pools ; but 

 ordinarily it is fairly smooth and is partly covered by organic 

 growths, which are more abundant near the seaward edge. 



The material is a sandstone that contains quartz grains, with 

 occasional beds of pebbles, a little mica, and shells and other 

 organic remains in varying amounts, all being firmly cemented 

 together by lime carbonate. The material is mainly so hard 

 that it rings under the hammer and fractures across the cruartz 

 grains and pebbles ; but in some places it is relatively soft. 

 The shells appear to be those of kinds at present living in the 

 adjacent water, and the other constituents of the rock are those 

 of the present beaches. 



Mode of formation. 



Difference in hardness seems to be the only important 

 feature that distinguishes the material of the reefs from that 

 of the beaches. The conclusion, therefore, is that the reefs 

 are beaches or sand spits lithified in place. This origin of the 

 reefs and their mode of formation was worked out by Dr. 

 Branner after a detailed study of the various reefs south of 

 the port of Rio Grande do Norte. The various phases of the 

 problem are treated fully in his publication on the subject,* 

 and his conclusions can only be summarized here. The main 

 features brought out by his study are as follows : 



The reefs are in reality hardened beaches or spits, that have 

 been encroached upon from both sides by the sea. Their 

 straightness implies that the original beaches or spits were 

 straight, and seems to be well accounted for by the usual 

 process of straightening of an irregular shore line by the cut- 

 ting down of headlands and the filling of embayments. The 

 natural features of the coast, notably the lack of good harbors, 

 the presence of low headlands which protect shallow bays, 

 and of stream mouths partially or wholly closed by sand bars, 

 are some of the factors which point to this conclusion. The 

 reefs are therefore believed to be portions of mature, nearly 

 straight, beach lines. The lime carbonate that hardened these 

 beaches appears most probably to have been dissolved from 

 calcareous beach sands by streams entering behind them, and 

 to have been redeposited along the zone of contact with the 

 salt water as the fresh water percolated seaward through these 

 *Bull. Mus. Comp. Zool., Geology, vii, 1904. 



