HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 218 
The evidence summarized indicates that the volcanic extrusions of 
the Isthmian section occurred largely during or at the close of Eocene 
time, although it is probable that.some took place in the preceding 
Cretaceous epoch. Although igneous action may have continued through 
the Eocene into later epochs, the sudden disappearance of igneous débris 
in the Upper Oligocene beds is against the latter conclusion. The 
Eocene was undoubtedly the period of time when igneous phenomena 
were most actively effective in the Isthmian region. We will show that 
the Tertiary series in adjacent Costa Rica, however, has been protruded 
through by still later eruptions, as shown by the studies of Gabb 
and Sjögren and the writer, and that these Post-Tertiary eruptives 
of Costa Rica are largely of different petrographic species from the 
older Isthmian rocks. 
The Post- Oligocene Folds of the Antillean Mountain System. — The 
sedimentation of the Eocene and Oligocene epochs was followed by 
great deformation, resulting from orogenic folding, the axes of which 
are in east and west directions, and which were probably accompanied 
by the deep seated “ syenitic" intrusions. 
The time of this folding, by analysis, can be approximately located 
as follows. The sediments composing the disturbed strata were in 
process of deposition in Eocene and Oligocene time, hence this folding 
must have taken place at а subsequent period. Likewise, it is apparent 
that the present horizontal Pleistocene sediments are deposited uncon- 
formably against the previously folded Oligocene strata. Hence the 
period of mountain making must have preceded the Pleistocene. There 
remains, then, only the late Miocene and Pliocene epochs to which this 
folding can be assigned. The absence of sediments belonging to either 
the late Miocene or Pliocene epochs on the Caribbean coast of Panama, 
and the evidences of long erosion with probable elevations during these 
intervals, force the conclusion that the time of the disturbance must be 
assigned to one of these sub-periods, or possibly both. The occurrence 
in Costa Rica of Pliocene sediments along the coastal margin, uncon- 
formably against the older folded Tertiary land, leads to the final con- 
clusion that this epoch of mountain folding must have been in late 
Miocene time. 
Erosion and Base Levelling of the Later Tertiary Epochs. — The 
evidence indicates that the Tertiary beds suffered much denudation and 
base levelling during the interval between the great orogenic revolution 
of late Tertiary time and previous to the swamp level sedimentation 
of the Pleistocene epoch. This erosion epoch was marked by the cut- 
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