D.- IB

2

Outline of Geology. There are four geological formations, near the power-house, which in downward order are — (1) Unconsolidated sands and. gravels, often 50ft. thick; (2) Much-jointed columnar rhyolite tuff, 50ft. thick: (3) Massive pumice breccia, 90 ft. to 100 ft. thick ; and (4) Tuffaceous clays, sands, and breccias, which extend to an unknown depth below the river. The first group of beds, the unconsolidated sands and gravels, here need no further consideration. The second and third formations are relatively strong rocks. They are porous rocks and when dry readily absorb water, but are not readily permeable, though the joints in them allow the passage of water. Weathering accentuates these joints, which are conspicuous on the faces of the cliffs. In the solid they are tightly appressed and often scarcely perceptible. Between the columnar rhyolitic tuff and the pumice breccia are a few feet of weak beds consisting of a poorly consolidated silty phase of the columnar tuff and thin beds of gravel, sand, and clay, the last mentioned being the soil of the gently undulating old land-surface of the pumice breccia on which the columnar rhyolite tuff was deposited as volcanic ash. These weak beds, especially the old soil, are decidedly impervious. The upper few feet of the fourth and lowest set of beds consists of indurated clays, probably the old soils of a former land-surface carved from weak tuffs and bedded sands. The power-house is built on these beds, of which only a small area is exposed. Indications of Deformation. Cracks. —The main crack ruptures the concrete at the corner where the spillway weir joins the penstock block and extends on the floor of the forebay nearly parallel with and close to the penstock block and along the headrace southward to a point 960 ft. from the north - east corner of the spillway. Northward, the small discontinuous fissures that appeared on the surface of the unconsolidated sands and gravels probably mark the extension of the crack in the rock below. These were traced for some distance, and, directly in line on the road from the low-level bridge to the transformer-house, several strong springs appeared at or near the base of the columnar rhyolite tuff. Cracks, more or less parallel with the chief break, cross the northern end of the penstock block. Another series of narrow discontinuous cracks in the covering beds extends at a slight angle from the main fracture, more or less parallel with the lower part of the overflow-channel, as far as the gorge eroded at its end. Strong leaks appeared at or near the base of the columnar tuff in this locality also. A table of measurements prepared by the engineering staff shows how the fissure in the concrete —the only place where it could be exactly measured —at first widened, and then, as the dam was emptied and the forebay drained, became, progressively narrower, until now it is less than half the width it was at the time of the maximum opening. The fissure in the forebay also showed progressive narrowing from the time it could be observed, and the fact that fragments of rock and pumice are pinched in it shows that decided closing occurred. Leaks. —The first indication that something untoward had happened was the flooding of the battery-room in the power-house. This vvas afterwards found to be due to the partial blocking of a drain which, on that account, could not carry the discharge from the leaks that appeared in the rock wall at the back of the power-house. Other leaks were discovered during the day. The leaks are in three localities : (a) Those near the power-house are from joint-planes in the pumice breccia and were at several points in the cliff some 30 ft. or 40 ft. above the main floor of the power-house ; another strong leak was through a joint-plane on the lower part of the No. 4 penstock tunnel. (b) Two decided leaks came into existence near the base of the columnar tuff on the road above the power-house and several smaller seepages at the same horizon in the cliff behind the power-house and in No. 4 penstock tunnel, (c) Five leaks were noted a little above water-level in the gorge excavated by the fall at the end of the overflow-channel ; these are at the same impervious horizon at the base of the columnar tuff. It should be noted that several of the leaks at first discharged milky water, but, in general, this discoloration disappeared within twenty-four hours. The lowering of the water in the forebay had also a decided effect on the amount of water yielded by all the leaks and seepages, as is well shown by the table prepared by the engineering staff. The leaks ceased altogether shortly after the water had drained from the forebay, though at that time the water still stood high in the dam. Tilts. —No. 1 generator had been levelled on the day preceding the formation of the crack, and, on relevelling, was found to have a slight tilt toward the river. The other generators, when the turbines stopped, were found to have tilts of similar amount, also toward the river. The 70 ft. tower at the west end of the suspension-bridge over the gorge was found to be 1| in. out of plumb, the inclination being toward the river. The ropes forming the handrails were noticeably slack, and, on measuring, the span was found to have been reduced by an inch or two. The tilts in the power-house and of the bridge tower steadily decreased as the forebay was emptied. Results of Resurvey. By survey, the power-house has been found to be slightly displaced ; but the precise amounts and directions of the movement have not been closely considered. The cracking and tilting above described could all be readily explained if the block of country between the spillway and power-house had tilted toward the gorge about one minute of arc, and indicate a maximum movement of the top of the block in that direction. If the rocks forming the block were absolutely rigid, the base of the block would be about 200 ft. below river-level.