HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 213 



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 debris 

 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 Sjogren 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 a 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- 

 vol. xxviii. — no. 5. 5 



