190 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Chin, Das Obispo, Haut Obispo, and Las Cascadas, where the Culebra 
basin begins. 'The outerops of basic igneous rocks south of this point 
of the Pacific are all different in occurrence and in character, and, as I 
shall show, probably belong to a different period of intrusion. 
The massive igneous rocks in situ of this subsection (see Plate V. 
Fig. 1), as above explained, exposed in the deepest cuttings along the 
lowest levels, and covered north of Bas Obispo by the great accumula- 
tions like the Mata Chin boulders, are of the greatest interest in the 
Isthmian history. At Mamei, in the cuts of the railroad at the base of 
a high hill in the south edge of that town, is seen the first of the mas- 
sives encountered in crossiag the Isthmus. This is of a basaltic nature, 
but apparently a tuff. Unfortunately, no specimen from this exact 
locality was obtained for petrographic study. 
At Mata Chin another outerop of black massive igneous rock is 
seen beneath a vast bluff of the unconformable Nigger Head (Mata 
Chin) formation. This rock, according to Professor Wolffs determina- 
tion, is basalt. From Bas Obispo to Las Cascadas the railway and canal 
run through a narrow gorge composed almost entirely of all the igneous 
rocks, both massive and pyroclastic. At Bas Obispo these consist of 
tuff, containing fragments of basic eruptives amd consolidated by a 
siliceous cement. Less than one tenth of a mile to the southward 
another mass consists of badly decomposed olivine. basalt or mela- 
phyre. Half a mile beyond the composition is a tuff of augite por- 
phyrite. Within a hundred yards to the southward the material is 
decomposed augite andesite. Entering Las Cascadas one mile to the 
southward, the material is a reddish silicified tuff with fragments of 
eruptives, immediately succeeded by reddish trachite. One tenth of a 
mile beyond, in the Upper Falls of the Obispo River at Las Cascadas, 
the stream flows over eruptive augite porphyrite. 
These numerous exposures are apparently at present one continuous 
mass of igneous material, but from the diverse mineralogic and physi- 
cal composition it is very evident that it is neither homogeneous nor 
synchronaus in all its parts, but represents an igneous area in which 
successive intrusions and eruptions have taken place, in time so long 
past that the whole — porphyrites, andesites, tufis, and basalts — have 
been rotted, decomposed, and sometimes recemented into apparent 
homogeneity. 
In no case does the original igneous material, although some of it was 
originally surface flows, appear to have formed the present topographic 
surfaces, but it is all apparently the remnant of very ancient intrusions, 
