252 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



of igneous rock in the basement beds of the Culebra clays, if the latter 

 be of Eocene age, testifies to the presence of basic igneous eruption 

 somewhere near the beginning of the Tertiary, if not prior thereto. 



The Tertiary strata of the Chiapas-Guatemala region, according to 

 Sapper, are iuterbedded with conglomerates of audesite, which must have 

 been extruded prior to the Tertiary period, while in other localities the 

 horizontal Tertiary strata are deposited directly on the andesites. 



There is but little doubt, as we have shown in the previous pages of 

 this paper, that during the early Tertiary period itself ti'emendous vul- 

 canism, probably accompanied by orogenic changes, took place ; in fact, 

 no doubt this was the scene of the most cataclysmic revolution of all 

 geologic time and place. We know that the older Tertiary strata else- 

 where than along the Isthmian section have been thrust through by 

 igneous intrusion. Theralites, trachytic, hornblendic, pyroxene ande- 

 sites, augite andesites, porphyries, and basalts have been pushed through 

 these sediments, shattering them by thousands of dikes, sills, and other 

 forms of intrusion. 



Theoretically, there is no reason to believe that some of these classes 

 of rocks did not exist before the Tertiary period. We know that at, or 

 just after, the close of the Cretaceous, the igneous rocks which mark the 

 whole American Pacific Cordilleran regions in later times were being 

 protruded in both the Andean, Central American, Mexican, and Eocky 

 Mountain regions, and the data submitted in this report show that this 

 action was taking place in the Isthmian region in early Tertiary time. 



The older basic eruptives continue eastward from the Isthmus, appear- 

 ing in trend with the Isthmus on the peninsulas of Goagira and Para- 

 guana, which guard the mouth of the Gulf of Maracaibo, and in the 

 outlying hills of Aruba, Curacoa, and Bonaire. They also appear along 

 the 10th parallel, between the 68th and the 66th degrees, in a narrow 

 belt south of Caracas. 



The "syenitic" and other deep seated granitoid rocks which have 

 been intruded into the Eocene Tertiary are certainly later in origin than 

 the basic eruptives of the Eocene epoch and older than the later basic 

 rocks of the volcanic plateaux. Their exact status in the Isthmian his- 

 tory is still somewhat involved, but they occupy no uncertain position 

 in the Antillean sequence, as will be shown in our report on Jamaica, 

 where they are directly coincident in age with the great Mid-Tertiary 

 epoch of Antillean mountain making. 



We also know that since the Pliocene epoch while the gigantic vol- 

 canic cones have continued to bury the surrounding structure with 



