274 GKOVE KAKL GILBERT-DAVIS tMjmWBa [v?£x3a* 



from hydraulic mining. The normal supply, coming from all parts of the drainage basin under 

 consideration, was estimated by examining the rate of waste from plowed fields, roads, trails, 

 and grazed lands, as well as from the much larger area of land under natural conditions; the 

 artificial supply was estimated from measurements of the volume of excavation in the aban- 

 doned hydraulic workings, reports previously made on this much-studied subject being 

 supplemented by Gilbert's own critical observations. The normal supply was found to be about 

 420,000,000 cubic yards, or nearly half of the total. 



Equal care was taken in estimating the present seat of the deposited debris. It is inter- 

 esting to note that 11 per cent remains in the mountains, 22 per cent is found in the piedmont 

 deposits, 4 per cent along the "valley rivers," 12 per cent in the river and delta marshes, and 

 49 per cent in the bays ; while only 2 per cent reached the ocean. Although the auriferous gravels 

 on the uplands were in large share of coarse texture, they were triturated so successfully dur- 

 ing transportation that the piedmont deposits are largely sand, and practically all of the bay 

 deposits, half of the total, are silt. The trenching of the piedmont fans was carefully studied on 

 the Yuba River, where many quantitative determinations were made. The encroachment upon and 

 the shoaling of the bay were quantitatively determined by comparisons of maps and soundings 

 of different dates; but preliminary to this a critical physiographic study was made of the entire 

 bay, with especial reference to the differential movement of the land on the two sides of the great 

 fracture known as the Hayward fault, by which the inner and outer parts of the bay are divided 

 and on which lay the focus of the earthquake of 1868. Although the evidence thus found of 

 slow movements up to a very recent date was held to be favorable to the view that subsidence 

 is still in progress, it is implied to be very small, for if it were rapid the area and depth of the 

 bay might thereby be increased in spite of inwashed detritus, and as a matter of fact they 

 have been decreased. It would thus appear that a subsidence of a significant geological rate 

 may be practically negligible in comparison with artificially accelerated aggradation. 



THE TIDES OF SAN FRANCISCO BAY 



In no respect was Gilbert's study made with more scrupulous care than where it touched 

 the tides of San Francisco Bay and the tidal bar outside of it; and in this respect the study is 

 the more to be marveled at, as that section of it was largely accomplished after his illness. 

 The volume of water alternately brought into and carried out of the bay by the tidal currents — 

 the so-called "tidal prism "—might, it would seem, have been fairly well determined by mul- 

 tiplying the area of the bay by the average rise of the tide; but Gilbert was far too cautious 

 and exacting to be satisfied by any such rough-and-ready method. He informed himself 

 minutely on the nature of the local tides, including the well-marked diurnal inequality of suc- 

 cessive high and low waters, the displacement of the times of rising and of falling slack water 

 from the times of high and of low water, and the slow progress of high water as well as the 

 decrease of tidal range inland through the irregular bay. The effects of the last two factors 

 on the volume of the effective tidal prism 2 as measured between the water surfaces at the 

 times of rising and falling slack water at the bay mouth was considered with especial care; 

 for curiously enough the departure of tidal oscillation from synchronism in various parts of 

 the bay results in diminishing the volume of the tidal prism, and hence in diminishing also the 

 velocity of the tidal currents at the bay mouth, below the values they would have if the bay 

 tides were everywhere alike in time and range; in other words, as the volume of a high tide 

 in the lower bay is supplied not only by the flood current which runs in through the Golden 

 Gate at the time of that high tide, but also in part by the ebb current from the preceding high 

 tide which is then falling in the bay heads, the volume of the flood at the Gate is thereby some- 

 what lessened. 



• There are a few lapses of verbal expression in the discussion of the "errective tidal prism" (pp. 71, 72), which may be ascribed to OUbert's 

 failing health, for such lapses do not characterize his earlier work. The unqualified statement at the outset: " During the rising of the tide, water 

 flows from the ocean into the bay; while the tide is falling, the flow is toward the ocean," needs a correction that is implied but not stated in the 

 lines next following; for the rising tide begins at the bay mouth while the ebb current is still running out, and falling tide begins there before the 

 flood current has ceased to run in. Farther on, the meaning would be clearer if the "effective tidal prism" were denned as the volume of water 

 which enters the bay "between any time of slack and the time of the following slack" at the bay mouth. 



