192 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the drainage profile of the Obispo increases, owing to the falls of tho 
river (aggregating 85 feet) which are located at this point. These falls 
are at the southern end of the narrow gorge marking the Mata Chin 
Section, as it cuts through the eastern perimeter of the circle of hills 
surrounding the Culebra basin, to be described. 
Tug CULEBRA SUBSECTION AND THE CULEBRA BASIN. 
From Haut Obispo to the Culebra Pass the country widens out into 
the great basin or amphitheatre, surrounded on all sides by massive 
hills not exceeding 500 feet in height, which are composed almost 
entirely of the basic igneous rocks." 
This basin, sub-oval in shape, has its greatest length north and south, 
'and sends out arms or embayments between the larger mountains, the 
Culebra Pass being one of these. This region is drained by the Obispo 
and Rio Masiniba. 
It is possible that this Culebra basin was once temporarily an en- 
closed lake, which has become drained by the cutting of a gorge through 
the volcanic rocks of the north border of the hilly perimeter between 
Haut and Bas Obispo. When I first ascended the Culebra Mountain, 
although ignorant of the fact that the Canal Company had formulated 
a plan to convert this basin into a lake by a dam at the lower end of it, 
I was impressed by the fact that here had once existed an interior basin 
similar to those found in many places in the irregular surface of the 
volcanic summit regions of Costa Rica, Nicaragua, and Guatemala, from 
which the drainage had escaped through the gorge between Cascadas 
and Mata Chin. 
The Culebra Olays. — The basin portion of the Culebra section is 
composed of stratified sedimentary clays and other sediments, having 
an important relation to the Isthmian geology. According to the 
borings of the Canal Company which I was permitted to examine in 
the office of the Chef de Section at Culebra, through the kindness of 
the Director General, these sediments at kilometers 53 and 55 in the 
Julebra Pass extend downward to at least 25 feet below sea level, the 
limit of the borings. They contain occasional seams of lignite, and I 
collected from the lowest cutting of the canal at Culebra Station frag- 
ments of fossil plants. These sediments occur between the igneous 
masses of the adjacent hills of the Culebra summit and Cerro de Lirio, 
1 This is the Culebra basin which it is proposed to convert into an artificial lake 
by a dam at Bas Obispo. (See Plate XI.) 
