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ANNALS NEW YORK ACADEMY OF SCIENCES 



formations, essentially a silicified dune deposit such as that at San Juan, 

 also by Tertiary limestone reefs, such as that at Quebradillas and at 

 Guanica, or by the still older igneous and clastic series, such as that at 

 Anasco, or by massive intrusives of a strictly igneous habit, such as that 

 at Maunabo. It would appear from this that the former outline of the 

 island must have been more irregular than it is at present and that the 

 distribution of marginal formations is also not as regular as has l)een 

 represented in earlier reports. 



Fig. 16. — Plaijn plain and marginal terrace 



View of the Pla.va plain (foreground), the marginal terrace (middlec lield) and tlie 

 mountainous divide formed h,v the Sierra de Cayey as seen from the "Central JMachete" 

 near Guayama. This terrace bevels across the upturned edges of shales, ashes and intru- 

 sives of the older series and is probably due to marine cutting. 



Terraces 



At many places on both sides of the island there are comparatively 

 smooth tracts having the appearance of bordering shelves which represent 

 true terraces. Their location along the sea margin and the compara- 

 tively insignificant development of similar benches along the streams 

 lead one to believe that they have an origin connected with the wave 

 action and attack of the sea. This interpretation is supported by the 

 presence of these terraces along the coastal margin where stream action 

 would not seem to have been able to reach. In any case, the presence of 

 such terraces, which stand from TOO to 200 feet above the present sea 

 level, indicate a former more submerged condition, so that the sea or 

 streams, or l)otli combined, were al)]e to attempt base-leveling at that ele- 



