HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 185 
coas formation. This brown material of the San Pablo character also 
outerops 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, 
and 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. 
Barbacoas Glass. Decomposed Rhyolite. 
California. 
Silica 58.13 51.02 
Potassa 1.24 48 
Soda .64 .22 
Lithia trace 
* 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. 
Tt is therefore not impossible that the Barbacoas glass is a true pumice (that ia 
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 med 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 
