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ANNALS NEW YORK ACADEMY OF SCIENCES 



tures coTild be developed even under a perfectly continuous but very slow 

 subsidence movement by reason of the natural seasonal ground-water 

 fluctuation. From this point of view, the range between two succeeding 



Fig. 15. — Detail of the iloiihJe structure in the San Juan formation at Arecibo 



Tills photogvapli Avas taken at the same point as the one shown in Figure 14 to bring 

 out the horizontal .structure crossing the inclined lajers. There is no doubt whatever 

 that the dark layer of less consolidated sand in the lower third of the photograph is a 

 primary bedding structure, but the horizontal marks crossing the inclined layers in the 

 upper part of the view are believed to be of secondary origin. 



