185 Costa Rica Miocene— Olsson 13 



In many parts of northern Costa Rica, the Gatim stage 

 carries at its base a thick and persistent conglomerate, formed of 

 coarse cobbles of igneous rocks, or of finer pebbles with alternat- 

 ing layers of sandstone. This conglomerate generally rests dis- 

 conformably on the Uscari, but in some cases is unconformable 

 on these or older rocks. This basal conglomerate is generall}^ 

 firmly cemented and because of its superior hardness, may be- 

 come an important ridge former. It marks the site of high 

 water-falls along man}-- streams. Fossils of common Gatun 

 types are abundant at many localities in this basal conglome- 

 rate. 



Gatun rocks are frequently found in areas which contain no 

 Uscari beds. In such cases, the Gatun is found resting, with 

 strong unconformity, on older rocks. This condition is seen in 

 the Canal Zone and along the borders of the sedimentary belt of 

 the western Panama. It indicates, either a complete erosion of 

 the Uscari shales during the Uscari-Gatun interval or, what is 

 more likely, a greater transgression of the Gatun sea. This 

 uncomformable relation of the Gatun upon older sedimentary 

 beds may be seen in the Chagres Spillway in the Canal Zone, 

 where the underlying rocks are sandstones, probabl}^ equivalent 

 to the Mt. Lirio of Lower Oligocene age. In the Chiriqui La- 

 goon country, of western Panama, the base of the Gatun is 

 frequently a coral-reef limestone, which rests directl}^ upon the 

 older and more disturbed rocks. 



The present systematic study enumerates 334 species of 

 molluska of which the majority belong to the Gatun stage. A 

 few of these species have not been found in Costa Rica or west- 

 ern Panama, but are common in the Canal Zone, and are includ- 

 ed here for the sake of comparison with Costa Rican species. 

 To these we may add the 18 species described by Gabb from 

 Sapote, Costa Rica, but not included in our collections and the 

 following species listed below from the Canal Zone. This brings 



