HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 201 



are embedded divergent feldspar laths, resembling the ophitic structure, except 

 that the green material does not have the same orientation throughout, but is 

 composed of optically independent grains. The interference colors of some 

 of this green material were too high for chlorite, suggesting that it may be 

 chlorite in which are embedded minute hornblende fibres. The exact nature 

 of this volcanic material is not apparent from this cursory examination, but 

 the appended chemical analysis of the freshest part of the specimen made 

 by Mr. Steiger indicates that it is probably of andesitic origin, unless it is 

 badly decomposed." The microscopic examination however indicates that it 

 is a soda-rhyolite-tuff or soda-trachyte-tuff. 



" Analysis of Volcanic Material from the Panama Cemetery Gate. 



(No. 45.) 



Silica 65.52 



Potassa 2.58 



Soda 1.99 



" No. 20, from Miraflores Station, five miles from Panama. From an out- 

 crop resembling disturbed and metamorphosed clays. This is a fine grained 

 reddish sedimentary (?) rock. Microscopic examination shows it to be com- 

 posed chiefly of minute grains that appear to be feldspar, with a few scattered 

 larger feldspars. Much of it is discolored by a white and yellowish decom- 

 position product of the nature of clay. It is possibly a feldspathic tuff. It is 

 evidently undergoing decomposition to clay." 



Professor Wolff says concerning this material, that " No. 20, Mira- 

 flores, and 23 and 36, Barbacoas, are the same. I have concluded that 

 they are composed of an acid rhyolitic pumice much like that found in 

 beds in some of the Western States. Also 37, San Pablo." 



From the report of Professor Wolff it will be seen that the Mira- 

 flores formation (which is the same as the Panama) strongly resembles 

 the Barbacoas formation upon the opposite side of the Isthmus, while 

 Mr. Turner likewise coi-relates them. Thus we have flanking both sides 

 of the central igneous section an extensive deposit of apparently related 

 formation quite different from the basic volcanic rocks. Upon the 

 Atlantic side the testimony is clear that this formation was made pre- 

 vious" to Bujio tuffs of the Tejon-Claiboime epoch of the Eocene fos- 

 siliferous sedimentaries, while upon the Pacific side the contacts show 

 that it has been pushed up, distorted, and broken through by the intru- 

 sion of the basic igneous rocks of supposedly Eocene age of the central 

 section. 



The composition of these rocks also probably indicates that they were 

 formed before the great basic igneous outbursts of Tertiary and later 

 times. They are largely made up of free quartz grains and feldspars, 

 such as do not occur in the later igneous rocks, and no formation since 



