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 foraminiferal 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 in 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. Тһе 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. 
Тнк 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. Тһе water gap of the Chagres through 
this region is a rapid succession of steep blufis, 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. 
The Mata Chin Formation (4 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 The term boulder formation if applied to these rocks might create confusion, 
because throughout Central America and the Isthmian region the term boulder 
