Page:Vol 6 History of Mexico by H H Bancroft.djvu/706

686 the morning, to explore the Chichihua River. We crossed the Tarifa River three times, the fir-st part of our journey being toward the north, and upon its valley a beautiful and grassy plain. At the end of the first three miles we left the Albricias Mountains and the Convento Cerro to the south-east, and began to ascend rapidly to the north, by the complicated system of hills which divide the Tarifa and Otate water-sheds.

I will be very particular in describing this, the last north-west spur of the Albricias chain, because here lies the heaviest part of the cutting for the canal-feeder.

The Albricias Mountain h made up of marbles and magnesian limestones, from the Convento Cerro till it reaches a point east of Santa Maria; it follows parallel to the Corte River for a short distance, and then turns east; in other words, compact limestone and dolomites skirt the base of the highest isthmian mountains, at a height of six or seven hundred feet above the ocean.

Due north of Tarifa are found soft limestones, very different from the dolomites above referred to, partaking more of the character of travertine.

They were deposited in strata, whose dip is now vertical, with a north and south strike, and this character is common to the whole of this spur; but the quality of its material varies a great deal, since sometimes it is calcareous tufa; then it passes into argillaceous marl, and again the clay disappears, and sand predominates.

This material is so soft that the foot-travel over it grinds it into impalpable dust.

The furrows made by each rainy season compel the Indians to leave last year's tracks for new ones, until they have made so many paths over these hills that it is confusing to select the best road.

North of this ridge the country descends to the Chichihua Valley, and is formed by soft sandstone, until the junction of the Chichihua and the Pericon, where jasper and argillaceous rocks are again seen ascending toward the north, forming the hilly district enclosed by the Chichihua, Malatengo, Corte, and Coyolopa rivers,

Occasionally plastic red and green clay are met with, as well as granular quartz bowlders.

South of the Pericon, the streams have cut their way through the rocks in situ, and neither drift nor indications of heavy floods are visible; but the bed of the Chichihua has clear, sharp sand brought down from the potreros east of Cofradia Range.

The summit of Sierra Blanca Pass is 1,232 feet above the ocean, and in order to get the waters of the Corte into Tarifa, it will be necessary to cut the thin web at this pass to a depth of 483 feet, or to tunnel through it, if it 13 found cheaper than either an open cut or a detour up the Tarifa River Valley. In the present state of my knowledge of the locality, I would propose a tunnel, that may be of small dimensions, through rocks that I know are very soft. This tunnel will be 9,650 feet long, and its area need not exceed that of a rectangle 19 feet by 12 feet, surmounted by a semicircular arch of 12 feet span. Under these conditions, its cost will be less than $600,000.