BERKEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO H 



Younger Series 



San Juan Formation. — On approaching the island by the usual route 

 entering San Juan harbor, the first rock whose structural detail can be 

 seen is that on which the city of San Juan itself is built. One can see 

 that the formation is made up of strongly bedded material that has all of 

 the structural characteristics of a cross-bedded sandstone and is resistant 

 enough so that it forms, at this point, a promontory extending far beyond 

 the supporting mainland and presenting a cliff face of at least 75 feet in 

 height. A closer inspection supports all of these observations as to 

 structure and adds the imioortant observation that the o-ranular material 



Fig. 2, — Partially destroyed dune sand deposits of the Sa)i Juan formation 



These deposits are a short distance west of Arecibo and are nearly covered with fresh 



dune sands of the same material. 



is, in large part, calcareous in composition and organic in primary origin. 

 The same type of material, with all of its characteristic structures, was 

 seen at several other points along the north coast. Special studies were 

 also made on sea cliffs of this formation in the vicinity of Arecibo, where 

 the exposures were so well developed that good photographs of the struc- 

 ture could be taken. One of these is reproduced as an illustration of the 

 characteristics of this formation. It shows exceedingly steep cross-bed- 

 ding structure that measures up to thirty-three degrees dip, and there are 

 also occasional structural lines that are almost horizontal. The rock is 

 exceedingly porous, the grains are unusually uniform in size, and the 



