BERKEY, GEOLOGICAL RECOXXOISSANCE OF PORTO RICO 41 



The island is comparatively abruptly terminated at both the east and 

 west ends. The yomiger limestone margin, which is fairly continuous 

 along the north coast and extends along about half of the south coa§t, is 

 wholly wanting at the east end and is also absent at the west end except 

 at the corners. At Fajardo, at the east end and at Rincon, at the west 

 end, for example, the older complex bedded rocks continue to the shore 

 line. In the uplifting of the present island mass, it would therefore ap- 

 pear that breaks occurred at both ends. The included mass is therefore 

 probably bounded roughly on three sides by faults, the east, west and 

 south, and is as a block tilted gently to the north. 



That there is, besides, considerable differential movement accompany- 

 ing the uplift and disturbance, is indicated by the warping of the erosion 

 plain lying beneath the younger series, the Eocene peneplain, which 

 stands essentially at sea level in the vicinity of Loiza and more than a 

 thousand feet above it at Lares. This difference is accompanied by a 

 much wider belt of these later limestones also in the region about Lares 

 than elsewhere. Such warping need not of course be confined to the last 

 movement: it may have accompanied the depression in eany Tertiary 

 time, permitting, as is indicated by deposits, very marked differences in 

 the development and encroachment of the organic accumulations. 



Large Structural Groups 



Where rock formations or field units are so numerous and so closely 

 related and so complex in primary structure if taken in detail, it is ad- 

 visable to combine them into fewer more generalized groups. A first step 

 of a very general sort, but in all respects sound, has been taken in recog- 

 nizing and using the terms '^Younger Series" and "Older Series" in this 

 paper. An additional step has been suggested in recognizing certain 

 smaller associations under the terms San Juan Formation, Arecibo For- 

 mation, Coamo Limestone Formation, Juana Diaz shales and marls, 

 Fajardo shales. Sierra de Cayey tuffs. Ponce chalky limestone, etc., but 

 these are for the most part local designations, some of which may well be 

 expected to become unnecessary after complete correlation is established. 

 A o^ood structural basis for sound subdivision of this sort is not vet 

 worked out. 



Unconformities 



There is only one marked unconformity in the structure of the island. 

 This is between the younger and the older series. It measures the break 

 in the sedimentary succession represented by the erosion interval during 

 which this mountain mass, now represented by Porto Eico, was reduced 



