266 BULLETIN OF THE 



river water was felt, which includes the delta region of coarser gravel 

 deposit. But it can readily be seen that certain physical sediments, 

 like tine gravel, will receive enough impulse from current and surf to he 

 carried into the margins of the salt water, so that unusually opportune 

 conditions are created for coral growth immediately where the water be- 

 comes sufficiently salt and food abundant. Thus it is that fringe reefs 

 do not usually form immediately at the mouth of rivers, but leave non- 

 coralline gaps in the reef simulating an extended submarine channel of 

 the river. This is clearly shown in the chart of Limones (Plate II. Figs. 

 1 and 2), and other rivers, where the present submerged fringe reefs 

 make a projected channel into the sea. 



It is the rule, whether the land is subsiding, rising, or stationary, that 

 the sea always indents the mouths of rivers after they have once reached 

 its level, and tends to wear away the angular points bordering its mouth. 

 This wearing is produced by the diurnal change of tidal level, and the 

 resulting constant corrasion, however small, of the bottoms, whether by 

 fresh or tidal current ; so the level of the sea, even in a delta-making 

 stream of perceptible age, will constantly encroach inland and cause 

 small estuarine deposits in the indented mouth at high tide, to be 

 moved outward with the ebb. Thus it is that the steep rivers of Cuba, 

 which are all very old and permanent, have slightly indented base level 

 with deposits of gravel extending inward coincident with the fluctuation 

 of the tide. 



The playa deposits found along the interior border of the harbors of 

 Havana and Baracoa represent the coarser gravel and silt thus formerly 

 given lip by the rivers upon reaching tide level, before the latest eleva- 

 tion. In the present Yumuri of the east the flood tide extends a mile 

 or less up the river. At the time of the general elevation of the coast 

 reef, the older delta deposits similarly formed were elevated correspond- 

 ingly, and are now found surmounting the lowest terrace. 



Snch an elevation as has taken place, and has produced the elevated 

 coast reef, would raise the present growing reefs above the water, so that 

 they would form indurated points at each side of the river's moutli, and 

 if there were a barrier reef its elevation would convert the old inside 

 deep into a land-locked harbor, while the old indented gravel would 

 form playas at the back of the harbor. Probably this is what has taken 

 place. Furthermore, the sides, of the narrow necks and sea fronts of the 

 harbors are composed of a harder and more durable stratum of reef rock 

 than the country back of them. The beach-like sides of the harbor 

 within the reef-like points are subsequently widened by undermining, as 



