BEREEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO 47 



horizontal markings would measure the fluctuation range of the ground 

 water, the harder zones representing in that case the more persistent 

 upper level for each succeeding depression position. 



The cross-bedding structure shows as plainly as it does on exposed sur- 

 faces also because of the fact that certain streaks are more perfectly in- 

 durated than the intervening ones, and the objection might well be raised 

 that a history of the kind suggested above woidd not be expected to de- 

 velop such a difference of induration in layers inclined at such high 

 angles to the horizontal. As a matter of fact, however, these sands are 

 not simple in their makeup. They are in large part fragments of organic 

 material and complete shells of small organisms of a calcareous nature 

 and the primary cross-bedding structure represents an assorting action 

 on this mixture of silicate and carbonate mineral material. It so hap- 

 pens, therefore, that the successive structural units are not necessarily of 

 the same mineral proportions, and in the process of induration, or of 

 binding the grains together, certain streak's yield more readily to this 

 influence and develop greater solidity and resistance to destruction. It 

 happens, furthermore, from a variety of rather unusual primary condi- 

 tions and secondary influences that both a primary and secondary struc- 

 ture of unusual prominence and peculiar association are developed in the 

 San Juan formation. 



SPECIAL RELIEF FEATURES 



Playas 



The flat areas along the coastal margin which are known as playas are 

 all developed at the mouths of rivers and are essentially alluvial deposits 

 of floodplain and delta type. In most cases they seem to occupy areas 

 that must formerly have been embayments in the coast. This develop- 

 ment is most striking, for example, at the mouth of the Arecibo and of 

 the La Plata and T.oiza rivers. In some cases, however, there is no 

 marked embayment and the deposit is more strictly marginal, such, for 

 example, as the Fajardo Playa at the east end of the island and others 

 on the south coast. 



Promontories 



In addition to tliL' embayments and playas, there are, occasionally, in 

 the intervening spaces, promontories where the rock formations extend 

 into the sea and terminate in cliff forms. These are neither numerous 

 nor are they confined to any particular portion of the island or to any 

 rock formation. They are represented by the most recent of all of the 



