turns, and applying the rise per mile of the river between San Miguel and Scarce's rancho, which are points well determined, we should find that the Chicapa, at eight miles from the rancho Scarce, must fall about 422 feet.
It needs no demonstration to prove that the Chicapa Valley is the only route for Moro's feeder.
At San Miguel this river is over 369 feet below the summit, and at El Palmar it is 24 feet above; hence the cutting below the Chicapa bed must commence 2,400 feet before reaching the Palmar Brook. This cutting, gradually increasing in depth, will be eleven miles long and 3,245 feet deep by the time it reaches that point of Aguas Nuevas Brook, from which we turned away to descend the Atravesado. But this is not all. There are fully one and a third miles of ascent from Aguas Nuevas Brook to the lowest point of Moro's Pass, and beyond this point the tunnel must extend through the base of the mountain, before reaching the Ostuta's bed.
The above supposes the Ostuta to be at a convenient height; but since it is 180 feet below the summit, besides the above cutting, 180 feet of depth of cutting must be added throughout the whole length of the feeder and summit. We became convinced of how useless was the attempt to explore any farther the source of the Ostuta, and taking additional barometrical observations, and cross-sectioning the river at the highest point visited, we turned back extremely disappointed.
The Ostuta delivers at the highest point 84 feet less than at Piedra Grande, or 119 cubic feet per second.
On the mountains north of Niltepec, the southern slopes are of clay; as we ascend we meet sandstone, compact limestone, and lastly gray and green slate, breccia, and porphyry.
Our next step, after leaving the Ostuta, was to try the San Miguel Pass, by way of the streams Chichihua and Pericon.
Exploration of San Miguel Pass. — We explored the San Miguel Pass, and the hasty study of its valleys convinced me that it was of importance to settle its practicability instrumentally.
Accordingly, while we were on our way to the Corte, orders were given to continue the transit and level lines from Tarifa to San Miguel via Sierra Blanca and San Miguel passes. Lieutenant-commander Bartlett was detailed for this work. The detour given to this line had for its object to study the range of hills dividing the Tarifa plains and the Chichihua potreros, in order to bring the feeder by this latter place in case the San Miguel Pass should prove impracticable. The work was performed by Mr Bartlett with the care and accuracy characteristic of this efficient officer. The San Miguel feeder-pass was found to be 1,071 feet above the ocean, and the Sierra Blanca Pass is 1,238 feet above the same plane. Although San Miguel Pass offers less height, the facts developed by the topographical and geological survey make Sierra Blanca the most feasible route for the feeder, for the following reasons: the Pita Brook is a forced point of pass of the feeder. In order to reach the San Miguel Pass, the feeder must turn south, up the valley of the Arroyo Corozo, and its grade will soon intersect the northern base of the Albricias Cerro, in a cutting of hard limestone and marble; it must then turn a right angle to the west, and skirt the southern lap of the Albricias Cerro, until it reached the Tarifa plains.
