183 Costa Rico Miockne — Olsson ii 



northern end of the Canal Zone and in the adjacent parts of 

 the Province of Colon. There the Gatun formation consists 

 of blue sandstones, dark-colored shales, fuller's earth beds etc., 

 generally less than 500 feet thick. Many of its beds are very 

 fossiliferous. The mollusks of the Gatun stage of Panama have 

 been studied by Dall, Toula, Brown, Pilsbry and Cossmann. 



In Costa Rica, the Gatun is very much thicker than in 

 the Canal Zone and represents a longer depositional period. 

 Paleontologically, the Gatun of the Canal Zone, seems to repre- 

 sent only the lower part of the formation as developed in Costa 

 Rica. It is largely of sandy character, more resistant to denu- 

 dation than the Uscari shales and gives rise to a more rugged 

 and hilly topography. Although typically sandy, it also contains 

 locally shales, lignites, conglomerates and coralline limestones. 

 In some localities the limj^ members are of major importance. 

 The formation is characterized b}^ marked lateral variation in 

 which respect it differs strongly from the much more uniform 

 shales of the Uscari stage. This feature of lateral variation 

 gives rise to two important phases: that of the m.ore typical 

 Gatun, in which the beds are largely sandy in character, with 

 some lignitic and coarsely conglomeritic members, and carrjdng 

 the usual near shore Gatun fauna; and the coralline phase, in 

 which the beds are more marine, with coralline limestone and 

 richly fossiliferous marls perdominating, and with no lignitic 

 beds. 



Along the inner margin of the Miocene belt, the Gatun be- 

 longs to the first and more typical phase; while the coralline 

 type is more common along the coastal zone, as at Port Limon 

 and Bocas del Toro. Coralline rocks may occur at any horizon 

 in the Gatun succession or it may locally be composed of mas- 

 sive coral-reef limestones. Such is the case in the Limon Penin- 

 sula, and such coralline limestones, by most observers, have been 

 regarded as of Pliocene or even of more recent age. At Limon, 



