from the shaft. At this point the rock dipped at an angle of 45°, and the gravels contained quantities of large boulders mixed with fine sand; immediately after encountering the gravel, a flow of about 90 liters per sec. was met, evidently coming through from a pot-hole in the shale. This quantity diminished in about 10 days to about one-fourth, but gradually increased again as the driving proceeded. The operations of driving the tunnel from 44 m. forward were begun in the dry season, in February, 1909, and the gravel was encountered for a distance of 24 m., or up to 68 m. from the shaft. The center of this gravel bed was about 30 m. south of the old river channel, which had been continuously dry at the surface for several years. Up to 68 m. the work was very difficult, owing to the upper part being of loose gravel and the lower in very contorted shale. The timbering of the tunnel in the full gravel section consisted of heavy square settings, 1 m. apart. At 68 m. the clay and gravel formation was met, and the rate of progress then was about 4 or 5 m. a week. A short branch gallery was also driven about 7 m. up stream near Shaft No. 2. The total distance the infiltration gallery was carried from Shaft No. 1, was 100 m., when the floods of August, 1909, caused its suspension.

During the progress of the gallery, attempts were made to sink a 31⁄2 by 2-m. shaft at a point along the line of the infiltration gallery, about 130 m. from Shaft No. 1, but water in such abundance was encountered that it was practically impossible to sink it in the ordinary way more than about 6 m. deep, the quantity of water to be dealt with amounting to about 20,000,000 liters daily. Seven shafts were then sunk in the high ground to the north of the river, five of these being on the line of the gallery and two 30 m. westward. They were sunk during the dry season prior to July, 1909. These were ordinary timbered shafts, 2 m. square between the walings, and were carried to the depths shown on [Plate XI]. In Shafts Nos. 5, 6, and 7 the water was flowing with considerable velocity, while Shaft No. 9 seemed to have penetrated a different water plane and one which was probably independent of that showing in any of the other shafts, in which the water was practically at a uniform level. The evidence obtained showed that if the gallery could be carried to Shafts Nos. 6 or 7 a great abundance of water would be intercepted. Owing to the difficulties of sinking ordinary shafts in the wide river channel, circular shafts were put down. These were 1.37 m. in internal diameter and 23 cm.

thick, and were of concrete reinforced with No. 10 vertical rods, 19 mm. in diameter, tied together with No. 6 wire. These shafts were provided with steel cutting edges.

Shaft No. 2 was sunk to a depth of 1 m. below the infiltration gallery level, No. 3 within 2 m., and No. 4 within 4 m., before August, 1909. The shafts were sunk by digging them out and loading them at the top, the top of the shafts being kept generally 3 m. out of the ground. Shaft No. 3 encountered great volumes of water, and, in order to enable sinking operations to proceed, a pumping shaft, 21⁄4 m. square, was sunk a little west of it to draw off the water. Notwithstanding the fact that the prolonged period of drought had lowered the general water plane in all the shafts to 565.00 m. above datum, the difficulties of handling the water even at that level were considerable. At the beginning of August the work was progressing very satisfactorily, but the extraordinary rainfall of that month caused the work to be shut down temporarily.

Effect of the Floods in the Santa Catarina River.—The area of the water-shed of the Santa Catarina River above Monterrey is about 1,410 sq. km. (544 sq. miles), and its area at San Geronimo, owing to its configuration, is practically the same. Its general character has already been referred to. On the night of August 10th and early on the morning of August 11th, a big flood came down the river, flowing at its maximum about 1,130 cu. m. (40,000 cu. ft.) per sec., due to the heavy rainfall [(Fig. 4)]. This flood carried away all the temporary staging around the shafts, seriously wrecking the temporary pumping station, as well as destroying the 30-cm. cast-iron pipe, notwithstanding the fact that it had been encased in a block of concrete 3 m. wide and 11⁄2 m. thick right across the river; but no damage was done to the infiltration gallery or to the shafts in the river channel. The effect of the flood on this pipe is shown by Fig. 2, Plate XXXI.

Plate XXXI, Fig. 2.—Profile Sketch, Looking Up Stream On Line Of 24-inch Main Supply Pipe.
[Larger.]

Following this flood, which had caused the loss of 14 lives in the city, 3 miles below San Geronimo, there was practically no rain for 13 days. Then, on August 25th the second heavy precipitation began and continued until August 29th, the details being shown on [Fig. 4].

This precipitation, therefore, fell on a water-shed which was completely saturated, as it had already absorbed a large proportion of the 13.38 in. of rain which fell during August 10th and 11th; and at every point along the river, prior to August 25th, springs were issuing forth,