HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 187 



Stratigraphically it is clearly sedimentary in nature, and water sorted, 

 and must represent the ejecta of some tremendous rhyolitic eruptive 

 epoch, preceding the time of the forarnini feral marls of the late Clai- 

 borne Eocene. 



The Barbacoas and San Pablo formations are not superficial, but very 

 old, as is shown by their dip in a direction that would carry them be- 

 neath all the rocks thus far described, and the fact that the bed of the 

 Chagres is now cutting through them. 



Their position indicates that they are far below the conglomerates of 

 the Bujio and Pena Negra hills, which iu turn underlie the Claiborne or 

 upper Eocene. The formation between the Barbacoas and Bujio forma- 

 tions is concealed by the jungle, but my impression on the spot was 

 that it might be the Culebra clays to be described. The Barbacoas 

 formation apparently continues as far south as Mamei. In my opinion, 

 based upon other facts to be given later, they were derived from igneous 

 loci existing to the southward, prior to the first recognizable beds of the 

 Eocene period. 



The Mata Chin Subsection. 

 (See Plate V. Fig. 1.) 



South of Bailo Monos begins the truly mountainous region of the 

 central Isthmus, and from that point (altitude 63 feet) to Cascadas 

 (altitude 165 feet) the river, followed by railroad and canal, passes 

 through deep V-shaped gorges, void of well defined benches between 

 the high, close set, conical hills. The water gap of the Chagres through 

 this region is a rapid succession of steep bluffs, composed of large spheri- 

 cal boulders of basic igneous rocks, which rest on protruding truncated 

 ends of masses of ancient massive igneous rocks and consolidated tuffs. 

 The hills through which the river cuts in this particular subsection are 

 the eastern periphery of the numerous eminences surrounding the Cule- 

 bra basin, to be described later on. 



Tlie Mata Chin Formation ("Nigger Head" Bluffs). — The great 

 bluffs of black basic rounded igneous boulders are called by various 

 names, such as " bomb-shell bluffs," " nigger head " bluffs, etc. Some 

 of the highest hills are composed entirely of these igneous boulders and 

 topographically resemble those entirely made up of massive rock, and 

 are ordinarily indistinguishable from them. The boulders are always 

 thoroughly rounded and apparently rolled. 1 



1 The term boulder formation if applied to these rocks might create confusion, 

 because throughout Central America and the Isthmian region the term boulder 



