396 



SCIENCE 



[N. S. Vol. LI. No. 1320 



mergence there was a progressive overlap from 

 west to east. Thus in eastern Porto Eico 

 and Vieques Island, the uppermost formation 

 of the Tertiary group lies directly on the 

 Cretaceous. 



Origin. — These Tertiary limestones have 

 been referred to as coral reef limestones. 

 This is misleading, for while corals are abun- 

 dant in the lowest reef limestone of the group, 

 the overlying limestones are made up chiefly 

 of foraminiferal and molluscan shells. 



The so-called "Pepino" or "Haystack" 

 hills (known as " Cock Pits " in Jamaica) are 

 not individual reefs or reef-mounds, as might 

 appear, but are the product of caving or 

 slumping caused by an extensive underground 

 drainage, aided by rapid surface solution. 

 The result is a peculiar type of karst topog- 

 raphy, seen on many of the islands of the 

 West Indies, but nowhere so well developed 

 as on the north coast of Porto Rico. 



Suhdivisions. — As a result of the work in 

 the Lares District, the writer has made the 

 following subdivisions of the Tertiary group 

 of the north coast: 



f Quebradillas limestone — 700- 875 feet 



. .-. Lob Puertos limestone — 550-1,000 feet 



AreeiDo j ^.^^^ limestone— 250-1,000 feet 



^^'■o^P Larea formation— 350-1,275 feet 



[San Sebastian shale— max. 700 feet 



In this classification, the names introduced 

 by Berkey^ have been used wherever possible. 

 The term "Areeibo," introduced by Berkey, 

 is used because the earlier name, "Pepino 

 formation," of R. T. Hill is a purely litho- 

 logical and topographical term, and is there- 

 fore vmdesirable. 



On the south coast, no detailed subdivision 

 has been made, but the names " Ponce " lime- 

 stone and "Juana Diaz" shale (basal mem- 

 ber) introduced by Berkey, are sufilcient. 

 After a careful study and comparison of a 

 large collection of Tertiary fossils from the 

 north and south coast formations, the follow- 

 ing correlation is made, and believed to be 

 essentially correct: 



* C. P. Berkey, op. eit., p. 14. 

 5 0. P. Berkey, op. cit. 



Age. — T. W. Vaughan,'' from a study of 

 fossil corals collected by R. T. Hill in the 

 upper San Sebastian shale and lower Lares 

 formation, concluded that the age of the 

 "Pepino formation" is Middle Oligocene 

 (Antiguan). C. J. Maury,'' from a study of 

 molluscan fossils collected in Porto Rico in 

 1914 by C. A. Reeds, concluded that the 

 Quebradillas limestone is of Lower Miocene 

 (Bowden) age, and that the " Rio Collazo 

 shale" (=San Sebastian) is Middle Oligo- 

 cene (Antiguan). The writer, from a study 

 of a large collection of molluscan fossils 

 from the Lares District, agrees with these 

 conclusions, but would place the Quebradillas 

 limestone (=> Bowden) in the Upper Oligo- 

 cene, rather than Lower Miocene. This de- 

 parture seems to be warranted by the abun- 

 dance of Orthaulax (several species) and 

 Ostrea antiguensis throughout the Quebra- 

 dillas. Furthermore, there is no faunal 

 hiatus or diseonformity to be foimd anywhere 

 within the Tertiary group of the north coast. 

 The entire series is a structural unit, as 

 Berkey pointed out.^ 



The ages assigned to the north coast forma- 

 tions are as follows : 



7. SaJi Juan formation Pleistocene-Eecent 



Diseonformity 



6. Quebradillas limestone (= Bowden) T Upper 



5. Los Puertos limestone j Oligocene 



4. Oibao limestone "1 Middle 



3. Laires formation 1- OUgoeene 



2. San Sebastian shale J (Antiguan) 



Unconformity 



1. Older Series Upper Cretaceous 



Bela Hubbard 



8 Bull. 103 U. S. Nat. Mus., p. 260, 1919. 



7 Am. Jour. Sci., Vol. XL VIII., p. 212, 1919. 



8 C. P. Berkey, op. (At., p. 15. 



