HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 185 



coas formation. This brown material of the San Pablo character also 

 outcrops in the bed of the river across from San Pablo. This formation 

 apparently extends as far south as Mamei. 



The Barbacoas rock was composed mostly of jagged fragments of sili- 

 ceous material which both Prof. Wolff and Mr. Turner pronounce to be 

 rhyolitic pumice. Black specks of basic igneous material are rare or 

 entirely wanting. Upon examination with a high objective I saw some 

 fragments which suggested that, while most of the material may be 

 pumice, there might be siliceous shells of diatoms among it. 



Specimens of the peculiar rocks of Barbacoas, Tavernilla, and San 

 Pablo, were submitted to Mr. H. W. Turner, of the U. S. Geological 

 Survey, who writes : — 



" The two whitish rocks from the Isthmus of Panama, one from Barbacoas, 

 aud the other from San Pablo (No. 37), are both composed almost wholly of 

 volcanic glass, in which are fragmentary feldspars, apparently in part sanidine, 

 but plagioclase is also present. The glass is extremely porous, and is in fact a 

 pumice in structure. It resembles very markedly a true pumice from Lipari, 

 except that there is present a large amount of a brownish decomposition prod- 

 uct, whitish and opaque in reflected light. A chemical analysis of the material 

 from Barbacoas was made by Dr. W. F. Hillebrand, and is as follows. 



" The Barbacoas volcanic glass is thus altogether too low in silica and alkali 

 contents to constitute a true pumice, but this may be due in part to decomposi- 

 tion. The analysis of a decomposed rhyolite collected by the writer from the 

 hydraulic washing one mile northwest of Volcano in Amador County, Califor- 

 nia, and analyzed by Dr. Hillebrand, shows an even greater loss of silica and 

 alkali. The glass groundmass of the California rock, however, is pretty thor- 

 oughly decomposed, but the abundant sanidine phenocrysts are little altered. 

 It ia therefore not impossible that the Barbacoas glass is a true pumice (that is 

 to say, a porous rhyolitic glass) which is somewhat decomposed. 



" No. 35, a white rock composed of rounded whitish particles cemented by 

 similar material, and labelled Barbacoas formation, below sed soil at Taver- 

 nilla. This is also composed almost entirely of volcanic glass, which is full of 

 gas or steam pores, many of which are drawn out in one direction. The 

 rounded particles seen by the naked eye are minute fragments of pumice that 

 appear to have been somewhat rolled about. These are cemented by porous 

 glass that contains numerous clear, broken, or imperfect crystals, apparently 

 feldspar. There is also some brownish decomposition material present, but on 



