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 voleanic 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. Тһе rolled 
débris 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 4 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 & 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 the presence of a great amount of contempo- 
raneously deposited eruptive material in their composition. These 
