Wallace Creek is an ephemeral stream in central California, the present channel of which displays an offset of 128 m along the San Andreas fault. Geological investigations have elucidated the relatively simple evolution of this channel and related landforms and deposits. This history requires that the average rate of slip along the San Andreas fault has been 33.9 ± 2.9 mm/yr for the past 3,700 yr and 35.8 + 5.4/-4.1 mm/yr for the past 13,250 yr. Small gullies near Wallace Creek record evidence for the amount of dextral slip during the past three great earthquakes. Slip during these great earthquakes ranged from-9.5 to 12.3 m. Using these values and the average rate of slip during the late Holocene, we estimate that the period of dormancy preceding each of the past 3 great earthquakes was between 240 and 450 yr. This is in marked contrast to the shorter intervals (-150 yr) documented at sites 100 to 300 km to the southeast. These lengthy intervals suggest that a major portion of the San Andreas fault represented by the Wallace Creek site will not generate a great earthquake for at least another 100 yr. The slip rate determined at Wallace Creek enables us to argue, however, that rupture of a 90-km-long segment northwest of Wallace Creek, which sustained as much as 3.5 m of slip in 1857, is likely to generate a major earthquake by the turn of the century. In addition, we note that the long-term rates of slip at Wallace Creek are indistinguishable from maximum fault-slip rates estimated from geodetic data along the creeping segment of the fault farther north. These historical rates of slip along the creeping reach thus do represent the long-term-that is, millennial-average, and no appreciable elastic strain is accumulating there. *Deceased. Finally, we note that the Wallace Creek slip rate is appreciably lower than the average rate of slip (56 mm/yr) between the Pacific and North American plates determined for the interval of the past 3 m.y. The discrepancy is due principally to slippage along faults other than the San Andreas, but a slightly lower rate of plate motion during the Holocene epoch cannot be ruled out.

Phase relations in a portion of the granite system, KAlSi3O8‐NaAlSi3O8‐SiO2‐H2O have been studied in the pressure range 4 to 10 kb. At 10 kb, melting begins at 625°C at an isobaric invariant point involving albite + orthoclase + quartz + liquid + vapor. The anhydrous composition of the liquid at this isobaric quaternary eutectic corresponds to 21 orthoclase: 56 albite: 23 quartz in contrast to that at the isobaric quaternary minimum at 0.5 kb, where the liquid corresponds to 31 orthoclase: 30 albite: 39 quartz. This represents a shift of the liquid, in anhydrous projection, almost directly toward NaAlSi with increasing P. The quaternary minimum in this system becomes a eutectic at approximately 3.6 kb. Compositional comparisons among granites, aplites, zoned pegmatites, and the quaternary minimum and eutectic suggest that igneous pegmatites and aplites represent magmas that were saturated or nearly saturated with water when they began to crystallize. Trends in bulk composition among pegmatites further suggest selective losses of alkalies during crystallization, probably by diffusion through aqueous fluids coexisting with the silicate melts and their solid products.

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The influence of the iron and ox/red conditions on the solubility of sulfur in water-saturated silicate melts.

The formations of central and east-central Vermont are exposed as a series of parallel belts that strike nearly north. Most of the rocks dip steeply, and many are overturned. With one possible exception, there seem to be no major repetitions within the sequence, and the order of formations from west to east appears to be the same as the order of their deposition. The formations are dominantly schist or phyllite, with varying proportions of arenaceous material. One thin formation, the Shaw Mountain, contains quartz conglomerate, calcareous tuff, and crinoidal limestone. The third-from-highest formation, the Waits River, is very thick and contains a large proportion of calcareous beds. The distance from the base of the lowest formation to the top of the highest, measured normal to bedding, is more than 100,opo feet; this large apparent thickness is believed to be not very much greater than the original thickness.The metasediments have been intruded by granitic dikes and plutons, mafic dikes, and small ultramafic plu,¥>;~-principal stages of deformation are distinguished. During the earlier stage the rocks were folded, and a schistosity was developed nearly parallel to bedding. Throughout the area the minor folds of this stage indicate a consistent upward movement of rocks on the east with respect to those on the west. The folds plunge at low to moderately steep angles, typically northward. Phenomena associated with the later stage of deformation decrease in intensity both eastward and westward from the belt underlain by the calcareous Waits River formation. At a distance from this formation, the rocks have prominent slip cleavage, and the earlier schistosity is folded. The minor folds plunge moderately to steeply northward on the western side of the area and more gently northward on the eastern. As the Waits River formation is approached, slip cleavage passes gradually into a schistosity that obliterates the earlier schistosity, and the intensity of later folding mcreases. In both the eastern and the western parts of the area the later minor folds indicate that the rocks of the Waits River formation have moved upward with respect to the formations on either side.The central part of the belt underlain by the Waits River formation is marked by a huge arch, 10-20 miles across, whose axis is more or less parallel to the belt and plunges gently northward. This is shown to be an arch, not in bedding, but in the later schistosity and in the axial planes of large isoclinal folds that were formed during the later stage of deformation. The axial planes of three of these large isoclinal folds can be correlated across the crest of the cleavage arch at Strafford Village.

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