194 bulletin: museum of comparative zoology. 



Cut 3. The same clays continue, but are cut by an intrusive sheet of igneous 

 rock. 



d. Sedimentary redeposit of igneous rock, dipping south, has fracture of newly 

 broken pig iron. 3 1 _ 34 feet 



e. Rotten, red, porphyritic looking clays, resembling the decomposed red por- 

 phyries in deep mines of Catorce, Mexico. It is evidently material of igneous 

 origin, which has been redeposited by sedimentation, and possesses lamination and 

 stratification. It also shows some indication of being metamorphosed clays. 



/. A persistent band of black oxidized material belonging to top of d. 



g. A band of yellow clay, conchoidal. 



Between d and e there is a thin band of small rounded pebbles. Total of 

 Cut 3, 20 feet. 



This section certainly looks as if d was intrusive into a and b, and that d is un- 

 conformable upon both, e is very much cross bedded. Prof. Wolff's examination, 

 however, shows that this igneous material is a sediment. 



Cut 4. The embankment of cut 4 is now nearly overgrown with grass, but in 

 places shows thin, black, laminated layers of clay shale, alternating with white 

 layers. The darker layers are very carbonaceous and full of plant stems. 20 feet. 



Cut 5. This is a mass of clay similar to cut 4. The clay is creeping down the 

 hillside. SO feet. 



6. Same as cut 5. 30 feet. 



Cuts 7 and 8 are towards the north (Colon) end of the mountain. They are 

 mostly deep red residual clay, with large angular blocks of the Culebra Summnt 

 igneous rock in them. These beds are apparently different from all the preced- 

 ing, and very much resemble the hillside boulder clays of Costa Rica. 



Above the clay bench (8) marking the north end of the mountain projects the 

 palisaded flat-topped crest of the Culebra Summit. This is a massive black pro- 

 trusion of igneous rock, and not a lava cap. According to Prof. Wolff, " This is a 

 coarse basalt or melaphyre, corresponding in structure to many of the thick trap 

 sheets of the Mesozoic diabases, palisades, etc. Its coarseness and lack of pores 

 make it probable that it is either an intrusive mass or comes from a very thick 

 flow." 



Whether the Culebra clays of the foregoing section are deposited 

 against the igneous mass of the Culebra Summit, or whether the igne- 

 ous mass was protruded through the clays, are questions which require 

 careful consideration. There are no suitable contacts exposed around the 

 Culebra mass which throw clear light upon this question, owing to the 

 fact that the subaerial decay is so extensive where the two formations 

 come together. The evidence of the sedimentary sills or beds of igneous 

 tuff and agglomerate interbedded with the clays in the lower portion of 

 the section is incontrovertible proof, however, that some igneous intru- 

 sion had taken place before the upper portion of the section had been 

 deposited, and I am inclined to believe that the intrusion of the Culebra 

 Summit rock was largely prior to or contemporaneous with the bedding 

 of the clays. The presence of the finely rolled, water- worn igneous 



