192 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the drainage profile of the Obispo increases, owing to the falls of the 

 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 eastei'n perimeter of the circle of hills 

 surrounding the Culehra basin, to be described. 



The Culebra Subsection and the Culebra Basin. 



From Haut Obispo to the Culebra Pass the country widens out into 

 the great basin or amphitheati'e, surrounded on all sides by massive 

 hills not exceeding 500 feet in height, which are composed almost 

 entirely of the basic igneous rocks. 1 



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 di-ained 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 Clays. — 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 

 t Culebra 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 Culehra basin which it is proposed to convert into an artificial lake 

 by a dam at Bas Obispo. (See Plate XI.) 



