Hershey. ] 



Isthmus of Panama. 



233 



Rica." * My study was made independently and without a knowl- 

 edge of his, and to a certain extent covered the same ground, hut 

 in the following pages I shall endeavor to avoid traversing matter 

 already treated at length by him, and add only new material to the 

 literature of Isthmian geology. 



The oldest formation observed by Mr. Hill on the Isthmus of 

 Panama is a water-laid rhyolitic and trachytic tuff, by him named 

 the Panama formation. I was fortunate in accidentally discovering 

 a series of formations under and consequently older than the Pan- 

 ama formation. Part of this series represents an old land which 

 once existed mainly to the southward of the present Isthmus. Mr. 

 Hill had suggested that such an ancient land occupied the position 

 indicated, and my happening upon a remnant of it was a remark- 

 able confirmation of his hypothesis. 



It is unfortunate that the strenuous conditions of my journey 

 prevented the collection and submission to paleontological special- 

 ists of sets of fossils from the pre-Panama formations encountered. 

 Marine fossils were observed in a number of places, although gen- 

 erally imperfect. My failure to secure specimens leaves this open 

 as a splendid field for some enthusiastic collector. Some of the 

 localities will be mentioned in the following pages. 



The absolute determination of the age of the earlier formations 

 must be left for future students, but I will ask the indulgence of the 

 reader to remarks showing what correlations are indicated by the 

 lithology, structure, and sequence of the strata. There is a remark- 

 able similarity between several formations on the Isthmus of Pan- 

 ama and a series of probably late Jurassic and Cretaceous age in 

 California. I submit that it is not a mere coincidence, but that 

 they have been formed under like conditions, at about the same 

 time, and subsequently subjected to about the same amount of 

 metamorphism. 



AREA STUDIED. 



The Isthmus of Panama, generally considered as coextensive 

 with that political division of the Republic of Columbia, officially 



* Published as Bulletin No. 5 of the Museum of Comparative Zoology, at 

 Harvard College, Vol. XXVIII. 



