44 



ANNALS NEW YORK ACADEMY OF SCIENCES 



Morro is built, but measurements of orientation were not taken there. 

 An occurrence of this rock immediately to the east ^ of Arecibo, a short 

 distance south of the lighthouse, showed structures of this kind on a 

 much larger scale than was seen elsewhere, and with an especially inter- 

 esting combination structure. The principal or stronger divisions are 

 widely separated and lie nearly horizontal. A comparatively small bed 

 lying in this position was almost unconsolidated, but those strongly cross- 

 bedded immediately above as well as those below were compact enough to 

 stand in a vertical cliff 30 to 40 feet higb. Tlie chief interest attaches 



Fig. 14.- — Cliff of the San Juan formation nontli uf the liyhthounc at Arecibo 



The prominent cross-bedding, extending throughout the upper thirtj- feet of the cliff, 

 Is well shown, together with a less strongly marked horizontal structure crossing the 

 same beds. The prominent break near the base is made by a layer of sand which is very 

 poorly consolidated. 



to the strongly cross-bedded portion forming the upper twenty feet or 

 more of the exposed cliff. The cross-bedding structure itself extends 

 without interruption through a much greater vertical range than in any 

 other outcrop examined, but its attitude and dip were not markedly dif- 

 ferent; the feature that was strikingly different from the structure seen 

 anywhere else was introduced by the presence of less pronounced but still 

 very plainly marked horizontal structures, making an interpenetrating 

 mesh-like arrangement in the face of tlie cliff. This can l)e seen strongly 

 enough to show in a photograph even at a distance of 100 feet. It is evi- 



