Hershev 1 



Isthmus of Panama. 



245 



Monterey formation (of Miocene age) developed extensively in the 

 Coast Range region of California, than any other of that state. 

 Both mark an epoch of very acid volcanic eruptions in their 

 respective regions. However, the erosion interval at the base of 

 the Isthmian Tertiary series hardly indicates a time sufficiently long 

 to cover the whole or even any large part of the Eocene period, 

 and we will, therefore, provisionally place the Panama formation in 

 the early part of that period. 



In all the formations which I have described, no clear evidences 

 of volcanic action of a modern character are to be seen until we 

 reach this last formation, which seems to be made up essentially of 

 fine ashes. To the epoch of acidic volcanic eruptions, there ensued 

 one of highly basic eruptions. The products of the latter occur 

 chiefly in the form of plugs, dikes, sills and laccolites of diorite, 

 andesite, basic porphyries and basalt which occur throughout the 

 area of the rhyolitic tuffs and are even scattered through the red 

 shale, basal conglomerate and the Santiago formation. These hard 

 rocks resist erosion better than the tuff and crop out like dikes and 

 bedded sheets, form most of the knobs and cap the peaks. Boulders 

 derived through their partial decay are scattered all over the vol- 

 canic region. 



It is a series of rocks of this character which forms the foot- 

 hills of the great range of the Cordillera de Veraguas, north of 

 Canazas and the Remanse gold mine, over a belt about 20 miles 

 wide. This was the region of chief volcanic activity, and in it the 

 deposits are of great thickness. Here the basic intrusives make 

 up a large part of the entire formation. In addition to the fine vol- 

 canic ashes, there are thick deposits of coarse gray ashes, much of 

 which has the macroscopic appearance of a massive rock and is not 

 water-laid. In fact, in this foot-hills belt the direct volcanic prod- 

 ucts of the rhyolitic and the basaltic epochs are so confusedly 

 intermingled that it seems advisable to consider them as a unit and 

 treat of them under the comprehensive term, the Panama Volcanic 

 Complex. 



This Panama series continues northeastward beyond Aguadulce, 

 between the narrow, low, coastal plain and the high sierra, and is 

 developed and well exposed in the vicinity of Panama. Between 



