12 



BUI.I.KTIN 39 



i84 



this confusion lias partly arisen from the fact that these Mio- 

 cene limestones are overlain by the second series, of Pliocene 

 a^e, which parti}' yielded the large fauna described by Gabb, 

 published shortly after his death in 1881. Dall later contribut- 

 ed to the discussion of this fauna. The truly Miocene limestones 

 of Limon may be seen in good exposures along the sea-front 

 west of the city. There they are massive in character, hard and 

 recrystallized to the extent that all traces of organic remains 

 have usually been obliterated or rendered uncertain of determin- 

 ation. However, some sandy or m^arly beds are occasionally 

 found intercalated in these coralline limestones, and from these 

 layers we have collected a large fauna of unquestionable Mio- 

 cene affinities, correlating closel}' in age with the Bowden beds 

 of Jamaica. 



The fauna of the Costa Rican Gatun is largely molluscan. 

 The following paleontological study records 334 species. Future 

 collecting w^ill doubtless greatly add to this list, as several species 

 common it the Canal Zone, still remains undiscovered in Costa 

 Rica. The smaller types of foraminifera abound in the more 

 shaly phases of the formation, as well as several species of echin- 

 oids, belonging to the genera Clypeaster and Encope. Corals of 

 simple and compound types occur principally in the coralline 

 phase. 



The: Uscari - Gatun Unconformity 



At the close of Uscari time the Miocene sea was with- 

 draw from the greater part of Costa Rica, but during the 

 Gatun, it returned. These conditions are indicated by a 

 stratigraphic break between the two stages, a complete change in 

 their lithologic composition, the occurrence of Gatun beds in 

 areas which had received no deposition during Uscari times and 

 a change in their respective faunas. Although the full strati- 

 graphic details of this important subject cannot be presented at 

 the present time, attention is called to it, as the outstanding fea- 

 ture of the Miocene stratigraphy of Panama and Costa Rica; and 

 because of its bearing on correlation. 



