HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 183 



The Barbacoas Subsection. 



The Bujio Formation. — The igneous tuffs near Bujio station (Colon 

 15.45 miles) constitute the range of high cerros lying back of the station, 

 and extend across the Chagres River in the direction of Pena Blanca (see 

 hills on background of Plate IX.). The Panama Railway Company has 

 been quarrying the side of this hill for years, and a vertical quarry face, 

 some 40 feet high, affords a good exposure for the study of the material. 

 This is composed of rounded and angular fragments of black massive 

 igneous rock (augite-porphyrite, Wolff) cemented by a matrix of decom- 

 posed brownish tuff. At first glance it has the aspect of being the 

 ordinary ejecta of a volcanic vent, but on closer study of its arrangement, 

 and the rounded character of many of the fragments, there seems every 

 reason for believing that it is a conglomerate which has been sorted 

 under water and arranged in banded and cross bedded structure. The 

 stratification planes have a slight dip to the northward, in harmony with 

 the general dip of the overlying fossiliferous sedimentaries. 



The Pena Negra hill, to the northward one mile, is in strike with the 

 Bujio hill, but is composed of a much coarser conglomerate, the material 

 assuming the proportion of boulders, two specimens of which were of 

 augite andesite. 



At Pena Negra the contact between the foraminiferal marls and the 

 igneous conglomerate was clearly seen. That the foraminiferal marls 

 have been deposited later, and against the Bujio igneous conglomerate, 

 has been shown by Mr. H. W. Turner, who reports that the former con- 

 tain some of the debris of the latter in its sedimentation, so that the 

 Bujio tuffs and Pena Negra boulders clearly represent volcanic debris 

 of contemporaneous or earlier origin than the fossilferous upper Eocene 

 Tertiary sediments. 



The Baila Monos Plain. — From Bujio southward to Baila Monos the 

 road follows a wide basin-shaped plain, sometimes three miles wide, sur- 

 rounded by low hills of the Bujio type. This is an old alluvial plain 

 of the Chagres, probably representing the lower portion of that river at 

 the period of the swamp level subsidence. Its mean elevation opposite 

 Barbacoas is 60 feet above the Caribbean. The low hills surrounding 

 this ancient valley or planation surface of the Chagres are composed 

 of a peculiar formation which will now be described. 



The Barbacoas Formation. — After leaving Bujio the railway runs 

 through a hill skirted plain, slightly rolling, and less swampy in char- 

 acter than those hitherto passed. No outcrops are seen until Frijoles 



