354 Prof. Hartt on the Brasilian Sandstone Reefs. [June, 



As a result of his careful study of the Pernambuco reef, and of 

 many others of similar character, some of which we will men- 

 tion farther on, Prof Hartt has given us the following conclu- 

 sions regarding the mode of formation of this class of structures : 



It is very evident that they are not the out-cropping edges of 

 beds of sandstone, extending out from the shore, as some have 

 supposed, but only narrow strips of stone of slight thickness, 

 formed in exactly the same position in Avhich we see them to- 

 day, that is, just below the level of high tide. They have resulted 

 from the solidification of beach materials, or sea beaches, by car- 

 bonate of lime carried into them by the percolating waters. This 

 action goes on from the level of mean high tide to a variable, but 

 only slight, distance below low tide mark, and has only a limited 

 horizontal extension. By the after encroachment of the sea, 

 aided by rivers flowing behind them, these consolidated beaches 

 have often been separated from the main shore as distinct reefs ; 

 but sometimes this latter action has not taken place, and the har- 

 dened layer retains its normal position upon the beach. 



The agencies concerned in the formation of these hardened 

 beach deposits are mostly very simple ones, many of which can 

 be witnessed by any person visiting the sea-shore. The slope of 

 a sand-beach varies according to the size and character of the 

 sand-grains composing it, the exposure of the coast and the 

 height of the tides. • When a wave strikes upon a beach it rushes 

 up the slope in a sheet of foam, carrying with it a quantity of 

 sand. As it returns the sand is spread out over the surface in a 

 thin layer. In this manner a regular lamination, dipping at a 

 more or less even angle, is produced in the niaterial composing 

 the beach. But this regularity is often much disturbed by 

 storms, when the beach may be broken into by the waves and 

 much of its material redeposited at quite different angles. 



If the land back of the shore be very low, the beach may form 

 a simple narrow ridge, over which the waves completely break at 

 high water, carrying and depositing sartd on the inner side of the 

 beach, where the dip of the laminse will of course be landward. 

 Ordinarily, however, a ridge of sand is formed behind the sea- 

 beach, above the reach of the tides, being partly due to the 

 action of the surf during storms, but mostly to the winds. Such 

 a beach-ridge as this accompanies most of the Brazilian beaches, 

 which latter, on account of the exposed character of the coast, 

 are formed of quite clean sand, consisting mainly of rounded 



