May 12, 1905.] 



SCIENCE. 



739 



is substantially limited by the planes of high 

 and low tide. The examples of these curious 

 structures hitherto described were in the im- 

 mediate vicinity of Pernambuco and along 

 the section of coast to the southward of Bahia. 

 The present memoir besides beautifully illus- 

 trating those already known with nearly 100 

 maps and photographs, gives full details of a 

 large number of others between the points 

 above mentioned and to the northward of 

 Pernambuco as far as Rio Grande do Norte. 

 The most southerly one known is at Gua- 

 rapary in the state of Espirito Santo and 

 from this point to Rio Grande do Norte, a 

 distance of about a thousand nautical miles, 

 the sandstone reefs can now be considered as 

 delinitely known. To the southward it is 

 tolerably certain that no more exist, but in 

 the nearly equally long stretch of coast from 

 Rio Grande do Norte to the Amazonas they 

 may be presumed to be about as abundant and 

 characteristic as in the section above men- 

 tioned, and it is much to be desired that this 

 northern section should be examined in the 

 same careful manner. 



Dr. Branner, after fully describing and il- 

 lustrating the reefs examined to the number 

 of over twenty, sums up their characteristics 

 as follows : 



The stone reefs are nearly but not quite straight. 

 The bedding of the material dips seaward at the 

 same angle as ordinary beach sands. The hard 

 rock of tlie reef is only three or four meters thick. 

 The underlying materials are sands, shells and 

 clays without regular sequence. The process of 

 formation, the character and the structure of the 

 reefs show that they are ancient beaches hardened 

 by lime carbonate, while their straightness shows 

 that they are forms of a mature beach line fixed 

 and made permanent by the process of consolida- 

 tion pointed out in Part VI. 



The most puzzling problem presented by 

 these reefs is that of their consolidation 

 through the deposition of lime carbonate, and 

 one of the most interesting chapters of the 

 memoir is devoted to its discussion. The 

 h^ioothesis of the hardening of beach sands 

 through the action of rain water and through 

 the escape of carbon dioxide contained in sea 

 water is admitted as possible, but put aside as 

 insufficient to account for all the phases of 



the phenomenon. The coincidence of the dis- 

 tribution of the stone reefs with an area of 

 greater density of the oceanic waters is noted 

 as a possible concurrent cause. (Another co- 

 incidence worthy of note is that of the dis- 

 tribution of the sandstone and coral reefs.) 

 A more efficient cause is thought to be the 

 seeping of fresh water charged with organic 

 acids from the decay of vegetable matter ac- 

 cimiulated in the lagoons and repressed 

 streams behind beach ridges. The reefs occur 

 along a section of coast swept by tolerably 

 constant winds and currents, and with such 

 geologic and climatic conditions that many 

 streams are temporarily or permanently closed 

 by the formation of beach ridges, so that their 

 waters, becoming charged with organic acids, 

 have to find their way to the sea by percolation 

 through the barriers of beach sands. In con- 

 clusion the author remarks : " It seems prob- 

 able that the consolidation of the reef sands 

 would not take place if the rainfall were large 

 enough and constant enough to keep the 

 mouths of the streams open and the waters of 

 the streams fresh" [pure]. 



In a chapter on coast changes evidences of 

 both elevation and depression in late geolog- 

 ical times are presented and discussed. A de- 

 pression of considerable importance is pre- 

 sumed to have taken place in early Pliocene 

 times, followed by a smaller elevation. The 

 sandstone reefs were formed and hardened 

 subsequent to the depression, but there is no 

 evidence that they have suffered any appre- 

 ciable movement since their formation. The 

 reefs, both sandstone and coral, have protected 

 the land and helped build out the shores. 



In an introductory chapter a brief sketch 

 of what is known of the geology of the coast 

 along which the reefs occur is given. In this 

 Dr. Branner takes issue with his predecessors, 

 and with his own previous writings, by refer- 

 ring to the Tertiary a considerable portion of 

 what has been considered as Cretaceous. The 

 evidence for this change of view is, however, 

 confessedly inconclusive, and to the reviewer 

 it seems that some contrary evidence of im- 

 portance has becu overlooked or unduly min- 

 imized. 



The chapter on coral reefs is also a valuable 



