HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 277 



fragments of hornblende, triclinic feldspar and augite from andesitic 

 lava. 



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



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

 Same as 36. 



(24) Cut in Railroad, a mile south of Culebra Station, Panama Rail- 

 road. Dirty gray tuff containing fragments of a glassy basalt. Another 

 slide is a fresh diabase of coarse basalt. 



(25 and 41a) Panama Canal, summit of Culebra. Dolerite (Basalt). 



(30) Panama Canal, Culebra Cut, No. 2. Gray tuff with fragments 

 of feldspar and basic (audesitic) lava. A gray agglomerate of small 

 fragments closely pressed together ; the cement is a fine cherty material 

 with round grains of black ore ; the feldspar crystals are often quite 

 perfect, but the rock does not appear igneous. 



(31) Panama Canal, Culebra Cut, No. 3. Dirty gray tuff with frag- 

 ments of feldspar and basic lava. Same cement as 30. 



(32) Panama Railroad. First hill west of Culebra station. Dirty 

 gray tuff with black basaltic fragments. 



(34) Culebra Cut, No. 3, Panama Canal. Gray tuff. The frag- 

 ments are of andesitic lava. 



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

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

 section is entirely 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 isotropic in polarized 

 light; contains curious round or elliptical bodies (lapilli 1 ?). 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 honeycombed with original glass inclusions. The cement is a green- 

 ish isotropic substance with frequent round or oval bodies, which are 

 now replaced by silica. Probably an altered rhyolite pumice. Much 

 like 23 and 36. 



(39) Isthmus of Panama. Savannah, northeast of Panama, in a small 

 hill. Tuff with andesitic fragments. 

 (41) Beach drift at Panama. 

 41. White porphyrite. 



41 bis. Reddish rock with plagioclase phenocrysts. A de- 

 composed porphyrite. 

 vol. xxviii. — no. 5. 9 



