186 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the whole the glass is much fresher than that of the specimens first described. 

 There is also present in the thin sections examined a ragged prism of greenish 

 brown hornblende with marked cleavage such as is sometimes found in coarse 

 hornblende-andesites, and two blue dichroic grains, which appear to be corun- 

 dum, and a squarish grain of iron oxide. 



" An analysis by Mr. George Steiger of the U. S. Geological Survey shows 

 this glass to be of rhyolitic origin, and as it is of the nature of a sediment it 

 may be termed a pumice-tuff. The preponderance of the soda over the potassa 

 would indicate that the rock is from a soda-rhyolite magma, and it will be of 

 interest to note' hereafter if soda-rhyolites exist there. 



" It is not safe to say, without much more extended information as to the 

 relation of these rocks in the field, whether or not they are all from the same 

 source, but the two first described, that from Barbae oas and the one labelled 

 37 (San Pablo) are pretty surely the same. The grains of corundum ? in 

 No. 35 may have come from an older schist series." 



The conclusions of Professor "Wolff, who independently examined these 

 rocks, closely coincide with those of Professor Turner. He says : — 



" 20, 23, and 36 are the same. They have bothered me considerably, but 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. 



" (20) Isthmus of Panama. Miraflores. Same as 23 and 36. 



" (23) Isthmus of Panama. Barbacoas crossing of the Chagres River. 



"(36) Barbacoas Formation at Barbacoas, Isthmus of Panama. In the 

 hand specimen a white kaolin-like mass, soft, which in the thin section is en- 

 tirely without action on polarized light. It may be opal ? The rock contains 

 fragments of feldspar and shells, which have been replaced by the silica. The 

 material is clear and iso-tropic in polarized light, contains curious round or 

 elliptical bodies (lapilli). It gives the test for a silicate and fuses rather 

 easily to a clear glass. Seems to be rhyolite pumice. 



" (37) San Pablo, Isthmus of Panama. A dirty gray marl-like rock with 

 green spots. Contains sanidine crystals and augite. The feldspar is honey- 

 combed with original glass inclusions. The cement is a greenish isotropic 

 substanee with frequent round or oval bodies, which are now replaced by silica. 

 Probably an altered rhyolite pumice. Much like 23 and 3(i." 



The reports of these two authorities clearly coincide in the opinion 

 that this vast formation, which must be somewhere between 500 and 

 1,000 feet thick, is a rhyolitic pumice and hence of igneous origin. 



