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University of California. 



[Vol. 2. 



here and the Culebra cutting on the proposed Panama Canal it has 

 been studied by Mr. Hill and by him named (excluding the intru- 

 sives) the Panama formation. Microscopic examination shows that 

 the early water-deposited portion in large part consists of a soda- 

 rhyolite tuff or a soda-trachyte tuff.* The basic intrusives and 

 their associated ash beds are strongly developed in the vicinity of 

 Culebra, where they include such types as "basalts (olivine, dia- 

 bases, and dolerites), augites (?) (both andesitic and porphyritic) and 

 trachyte tuffs of similar materials, and one boulder bluff of horn- 

 blende augite (?), andesite, or porphyrite." Mr. Hill also makes the 

 acidic distinct from and older than the basic eruptive epoch. 



Now, subsequent to the intrusion of the Panama formation by 

 these basalts and andesites, their debris was formed into the Bujio 

 conglomerate, which is the basal formation of a series of sedimen- 

 taries developed on the Atlantic side of the Isthmus. The age of 

 these formations is definitely fixed by abundant fossils. The range 

 is from the Claiborne stage of the Eocene merely as far as the early 

 Miocene, when sedimentation ceased in the Isthmian region and was 

 not resumed until the early or middle Pleistocene. On the strength 

 of this evidence, Mr. Hill correlates the basic volcanic epoch with 

 the late Eocene and considers the Panama formation proper of 

 considerably earlier age, probably Cretaceous. 



There is no positive evidence of the pre-Eocene age of the Pan- 

 ama formation. As a matter of fact, it is my impression that the 

 deposition of the rhyolitic tuff did not long precede the intrusion 

 of the basalts and andesites. As the latter, we now know, occurred 

 rather late in the Eocene period, we may reasonably place the 

 former early in the same period. We will then divide the history of 

 the Eocene period as follows : — 



1. A short epoch of erosion. 



2. An acidic volcanic epoch with partial submergence of land, 

 and deposition of Panama formation proper in sea on borders of 

 the volcanic range. 



3. A basic volcanic epoch, with intrusion of Panama formation 



* Determination by Turner from specimens submitted to him by Mr. Hill. 

 See paper by latter, page 201. 



