(SoO J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



the beating of the precessioiial clock, valuable clues may be obtained as 

 to the length of time required for the deposition of certain formations. 

 It should be looked for under conditions where seasonal changes might 

 be expected to atfect the vegetation, the regimen of rivers, or the intensity 

 of wave action in shallow seas. Such conditions are suggested in middle 

 or higher latitudes, where temperate faunas and floras exist or where the 

 deposits indicate the existence of a semi-arid climate. The evenly bedded 

 fine-grained deposits in lagoons or on the floors of shallow seas would be 

 more favorable than the irregular bedding of river plains. The deposits 

 of lake bottoms or .of fore-set slopes, or of sea-floors temporarily below 

 wave-base, are perhaps the most favorable situations. Transition regions 

 between two facies are most favorable, as where sheets of lime and clay 

 sediments are interfingered. In such places a slight shifting of climatic 

 factors is recorded by a more or less wide shifting of the border line 

 between the facies. The shallow limestone seas of the earlier periods 

 should be examined, for they show a marked oscillation in bedding, chang- 

 ing conditions in salinity, and varying turbidity of shallow waters. 



To .reach a conclusion an exposure should be extensive. The proof 

 would be to find a thin lamination marking an annual rhythm gathered 

 into higher rhythms, and these into still higher but fainter groupings, so 

 tliat a restoration of the annual layers would aggregate about 20,000 in 

 one of the larger and fainter groupings. This may seem difficult to attain, 

 and a more favorable, but less conclusive, recognition of the precessional 

 cycle may be sought in beds which probably represent each several cen- 

 turies or some thousands of years, grouped into regularly recurring but 

 faint rhythms, which shall comprise some tens rather than units or hun- 

 dreds of the lesser beds. There may have been times, however, when the 

 flow of solar energy was so smooth that the precessional rhythm could 

 emerge into control and find stratigraphic expression. It would be marked 

 by great regularity, but other rhythms may, however, rise into recurrent 

 dominance and exhibit for a time a marked regularity, so that caution 

 in identifying rhythms must be ever present. 



The possibility of measuring the time of accumulation of strata by 

 means of climatic rhythms has been dealt with rather fully, since not- 

 withstanding the lack of definite results thus far obtained, it seems to 

 contain large possibilities and should lie in the background of the menta] 

 vision ready to be invoked. 



Many stratigraphic sections show notably regular rhythms in the alter- 

 nations of beds. A few of the more conspicuous and regular may be cited. 



Two remarkable formations, the Sausalito and Ingleside cherts, occur 

 in the Franciscan group of probable Jurassic age.'^^ The former is about 



■^1 See A. C. Lawson : San Francisco Folio, U. S. Geol. Survey, No. 193, 1914. 



