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ANNALS NEW YORK ACADEMY OF SCIENCES 



varies very much in different parts of the field. Its most striking devel- 

 opment is in the district extending from" Tao Alto to Aguadilla. In the 

 district extending eastward from San Juan and also in some of the areas 

 on the south side, this margin is very much broken and so obscure in 

 some parts as to escape detection. In its best development, however, it 

 is a typical cuesta, formed in the usual manner by the erosion of a for- 

 mation representing a recently uplifted coastal series; The series of 

 formations involved formerly extended inland very much farther than 

 they do now. Only the outer margin remains from the erosional de- 

 struction of a series of beds and reefs that in former times covered a 



Fig. 18. — Inner lowland near Bayamon 



View looking north from tlie Bayamon-Comerio road toward San Juan, showing the 

 monotonous features of the lowland helt in the foreground and the coniparativelj- promi- 

 nent hill remnants of the Tertiary formation cuesta in the background. 



large portion of the island. The road running from Aguadilla to Moca, 

 San Sebastian and Lares extends for practically the whole distance, after 

 leaving the coast, along the inner lowland at the foot of this cuesta or 

 along the cliff forming the inface. The same features characterize the 

 surface topography as far east as Corozal. This feature is much less 

 pronounced on the south side of the island. 



Peneplain 



Beneath the limestones constituting the cuesta and representing the 

 Tertiary series there are, in numerous places, traces of a former plain 

 that represented tlie results of erosion on rocks that had a complex struc- 



