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ANNALS NEW YORK ACADEMY OF SCIENCES 



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

 subsidence movement l)y reason of the natural seasonal ground-water 

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



^ 



Fig. 15. — Detail af the iluiihlc strurtme in the San Juan formation at Arecibo 



This photograpli was taken at the same point as the one sliown in Figure 14 to bring 

 out the horizontal structure crossing the inclined layers. 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 secondarv origin. 



