210 bulletin: museum of comparative zoology. 



through atmospheric and marine erosion, nearly all been degraded to 

 marine base level. That the basic igneous rocks did not originate in 

 recent time has been shown. Nowhere on the portion of the Isthmus 

 visited by the writer did he find overcapping flows of the igneous rocks 

 resting upon the Tertiary or later strata, except in the case of those 

 in Panama Bay, and the connection of these with the central igneous 

 protrusious was not proven. While there is sufficient variation in the 

 character of these rocks to indicate that they represent more than one 

 outburst, their lithologic composition as a group is different from the 

 later volcanic ejecta of the adjacent region of Costa Rica, from whence 

 lavas have flown in Post-Tertiary time, as will be shown. None of the 

 newer and more recent-looking igneous rocks encountered in my journey 

 across Costa Rica were seen upon the Isthmus, although the older base- 

 ment igneous rocks upon which the later volcanic phenomena of Costa 

 Rica are built up are quite analogous. 



We have shown that these basic rocks were clearly pushed through 

 the older Pre-Tertiary Panama formation at Miraflores. The rolled 

 debris interstratified with Culebra clays show that the clays are newer 

 and later than some of the igneous rocks which had previously existed 

 in that vicinity. The immense beds of rolled igneous boulders found 

 immediately beneath the Vamos a Vamos beds certainly show that a 

 great mass of igneous rock existed before the latter were deposited in 

 late Eocene time. 



The occurrence of basic igneous material intermixed in the fossiliferous 

 sediments in the Eocene Tertiary rocks of Bujio and Vamos a Vamos, 

 and the unconformable position of the latter upon the stratified basic 

 igneous conglomerates of Bujio and Pena Negra, shows that these igneous 

 rocks existed during or prior to the deposition of these later Eocene 

 sediments. 



If the Empire limestone and the foraminiferal beds of Bujio, contain- 

 ing apparently the same fauna, are of approximately synchronous age, 

 then the difference in composition of the imbedding material gives a clue 

 to one of the great basic eruptive periods of the Isthmian region, for 

 between these outcrops occurs an immense thickness of volcanic boulders 

 and tuffs of the basic igneous rocks. The foraminiferal Empire lime- 

 stones are apparently free from igneous material, while the equally 

 foraminiferal deposits of the canal section, between kilometer 20 and 

 kilometer 25, lying upon the top of the interstratified volcanic rocks of 

 the Bujio distinctly show 4he presence of a great amount of contempo- 

 raneously deposited eruptive material in their composition. These 



