HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. ШТІ: 
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 6f 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 (andesitic) 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 Жей. The frag- 
ments are of andesitic lava. 
(36) Barbacoas Formation at Barbacoas, Isthmus of Panama. Іп 
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The rock contains fragments of feldspar and shells which have been 
replaced by the silica.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?). 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 isotropie substance with frequent round or oval bodies, which are 
now replaced by silica. Probably an altered rhyolite pumice. Much 
like 23 and 36. 
(39) Isthmusof Panama. Savannah, northeast of Panama, ina small 
hill. Tuff with andesitic fragments. 
(41) Beach drift at Panama. 
41. White porphyrite. 
41 bis. Reddish rock with plagioelase phenocrysts. A de- 
composed porphyrite. 
VOL, XXVIII. — NO. б. 9 
