190 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Chin, Bas Obispo, Haut Obispo, and Las Cascadas, where the Culebra 

 basin begins. The outcrops 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 crossing the Isthmus. This is of a basaltic nature, 

 but apparently a tuff. Unfortuuately, no specimen from this exact 

 locality was obtained for petrographic study. 



At Mata Chin another outcrop of black massive igneous rock is 

 seen beneath a vast bluff of the unconformable Xigger 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 asd 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 Biver 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 

 synchronous 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, tuffs, 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, 



