BERKEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO 51 



ture. Occasional profiles of more distant ridges also show a sky line that 

 suggests the former existence of such a plain, and in favorable localities 

 it can be traced directly to the foot of the limestone cuesta. Occasional 

 traces are also seen on more mountainous tracts, especially at the west 

 end of the island, near Eincon and in the vicinity of Mayaguez. At the 

 latter place, these remnants of the old pjain are called mesas. It is the 

 judgment of the writer that these all belong to a single base-leveling sur- 

 face or marine-cut platform formed in the period just preceding the 

 development of the Tertiary limestone series. Judging from beds devel- 

 oped immediately upon this surface, it must have been completed in early 



Fig. 19. — Haiistack (pepino) hills 



A characteristic view, showing the small soil-covered flats and associated haystack 

 hills found in the region of the Arecibo formation. Photograph taken on the road be- 

 tween Arecibo and Barceloneta. 



Eocene time, and perhaps was even largely developed in pre-Tertiary 

 time. It may be referred to as the early Tertiary base level or peneplain. 

 There are many minor features giving variety to the surface relief 

 which depend for their particular relations and character on underground 

 structures which are as yet imperfectly understood. 



Haystack Hills 



The most striking topographic feature of the whole island is the re- 

 markable development of small isolated or grouped rugged hills usually 

 rising abruptly above adjacent smooth flat soil-covered areas at various 

 levels throuo-hout a broad mar^final belt alone: the north coast, west of 



