12 ANNALS NEW YORK ACADE3IY OF SCIENCES 



binding material is calcareouSj attaching one grain to another merel}^ at 

 the point of contact. The extension of ledges of this kind of rock far 

 beyond the possible reach of swift-flowing streams, together with the fact 

 that the distribution is limited to certain sections of the north coast, and, 

 in addition, the evidence inrnished by the internal structure of the rock 

 itself, lead to the conclusion that the formation is essentially an old dmie- 

 sand deposit. Sand dunes are developed on the present coast line from 

 very similar material, but none of the very recent dunes are solidified. 

 On the other hand, the San Juan formation seems to have been developed 

 Ijefore certain of the later elevations and subsidences that affected the 

 island in its recent history, so that its material is fairly well cemented 

 and its base extends below the present water level. Its outcrops also ex- 

 tend to greater elevation above sea level than any of the modern dunes. 

 It is judged, therefore, that this particular formation is the most recent 

 of all in the island to act as a ledge former, and it is judged by its situa- 

 tion and content to be of Pleistocene age. See additional description 

 imder heading "Structural Features.^^ 



This is the most unusual formation in the island. It is a type seldom 

 seen or seldom recognized, and it is one of the smallest in Porto Rico, in 

 spite of the fact that it makes such an imjDortant showing at San Juan 

 harbor. 



Because of the prominence of the formation in the city of San Juan it 

 is suggested by the writer that the name San Juan formation be used for 

 it and that this name be confined to the Pleistocene beds representing 

 solidified sand-dune deposits. 



Arecibo Formation. — N"ext below and older than the San Juan forma- 

 tion is a great series of reef limestones and shell limestones preceded by 

 shales that form a belt of considerable width along the north coast and a 

 portion of the south coast of the island. In a large way this series forms 

 a structural unit. Above it in all cases lie the recent alluvial deposits 

 and the San Juan formation and below it lie the older and more compli- 

 cated igneous and sedimentary rocks. The break between these two rep- 

 resents the chief unconformity in the whole geological column. The 

 heaviest development of this formation is along the north coast between 

 Tao Alto and Aguidilla. In this belt, the massive limestones of the Are- 

 cibo formation attain the greatest thickness observed anywhere on the 

 island, but no opportunity was found for determining the amount accu- 

 rately. There is in sight, however, certainly as much as 500 or 600 feet 

 in the ]:)luffs along the Arecibo Eiver. In this belt also, especially farther 

 toward the west, in the vicinity of Lares and San Sebastian, there are un- 

 derlying shales of considerable thickness which in places carry lignitic 



