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University of California Publications in Geology [Vol. 12 



Survey show a northerly movement in the interval ; and the evidence 

 of this is more abundant on the east side of the San Andreas fault than 

 on the west side. This northerly movement is, in accordance with the 

 rebound theory, interpreted as an expression of strain. The mean 

 rate of northerly strain creep of the region for the period between 

 surveys I and II based on the observations at Tamalpais, Chaparral, 

 Ross ]\Iountain, and Farallon is .052 meters per year. 



2. The results of observations at Ukiah undertaken since 1900 to 

 establish the variation of latitude, show that, besides the rhythmical 

 variation due to the movement of the pole of rotation around the 

 pole of figure of the earth, there is a northerly migration of the station 

 itself. I am not aware that this fact has been recognized by the 

 astronomers; but certain published results clearly show this to be a 

 fact. In a i^aper by Sir P. W. Dyson* on "The Variation of Latitude" 

 there are published curves showing the variation of latitude, 1900- 

 1917, fit Ukiah and other stations. The curve for Ukiah is here repro- 

 duced, on a different scale, in figure 1, A. It shows the periodic depar- 

 ture of the station from its mean latitude, or mean position of the pole, 

 as determined from observations at several stations around the earth 

 established for the purpose. If we consider the peaks of the curve 

 on either side of the zero line, it is apparent that there is a pronounced 

 tendency for them to migrate northerly in the interval covered by the 

 cui've. I have endeavored to bring this out by regarding the peaks 

 on each side as falling into a curve and smoothing out the curve as a 

 straight line. Owing to the variation of amplitude of the swing of the 

 pole some of the peaks depart somewhat from the sloping lines that 

 I have drawn, but this departure is more or less symmetrical on both 

 sides of the zero line and is clearly due to a periodicity in the ampli- 

 tude. The sloping lines will I think be conceded to be a fair expression 

 of what appears to be a regular northerly migration of the station 

 at Ukiali. The upward slope of the upper line, embracing the peaks 

 of the northerly departure from the mean, is more pronounced than 

 the lower line embracing the peaks of southerly departure. Taking 

 these two lines as significant of a northerly creep of the region and 

 basing my figures on the mean of the two sloping lines, I find that 

 Ukiah is creeping northerly at the rate of about .29 meters per year. 

 In arriving at this figure it is to be noted that, since the mean position 

 of the pole represented by the zero line is based upon observations 

 at all the stations including Ukiah, the zero line adopted diverges 



8 Monthly Notices of the Eoyal Astronomical Society, vol. 78, no. 7, May, 1918. 



