HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 183 
THE BARBACOAS ŠUBSECTION. 
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 1Х.). Тһе Panama Railway Company has 
been guarrying the side of this hill for years, and a vertical guarry 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 débris of the latter in its sedimentation, so that the 
Bujio tuffs and Pena Negra boulders clearly represent volcanic débris 
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 
