832 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



sands upon the siliceous deposits. These sands now form the Marin sandstone. 

 After this sandstone had accumulated to a thickness of about 1,000 feet the 

 subsidence of the sea-bottom, which had been in progress during the period of 

 its deposition, caused the shore again to retreat so far that the conditions be- 

 came once more favorable to marine life and another deposit of radiolarian 

 ooze was laid down. This ooze formed the Ingleside chert. Again uplift set 

 in, shallow water prevailed, the shore was close at hand, and the sands of the 

 Bonita sandstone were deposited." 



These cherts would seeiii to be deposits made below wave-base, and 

 although the rhythm is probably due to shiftings of currents, this in turn 

 has doubtless a climatic cause. The rhythms are apparently, however, 

 too numerous within a limited geological time to be due to the preces- 

 sional cycle, even for the long estimates of time given by radioactivity. 

 They suggest, therefore, the presence in the Jurassic of solar rhythms 

 similar to those of the Pleistocene, but in a period of wide-spread equable 

 climates. 



Gilbert^^ has called attention to a notably regular alternation of strata, 

 recurring four times in the section of the Cretaceous exposed in the basin 

 of the Arkansas Eiver near the Rocky Mountains. This is found in the 

 shales of the Benton, Niobra^^a, and Pierre groups, totaling 3,900 feet 

 in thickness. The pronounced rhythm is seen, however, in only a small 

 proportion of the total thickness. 



The rhythm of shale and limestone is 1.5 feet thick where the prin- 

 cipal deposit is limestone, and repeats itself every 2.7 feet where the 

 calcareous material suffices only to modify an otherwise argillaceous shale. 

 Allowing a somewhat more rapid rate of accumulation for the more 

 uniform and non-calcareous portions of the series, Gilbert takes 4 feet 

 as an estimate for the time interval of the rhythm through the whole 

 3,900 feet. The only regular rhythm of sufficient length known to Gilbert 

 at that time was the precession cycle of 21,000 years, which gives what 

 would generally be regarded as the excessive estimate of 20,000,000 years 

 as the length of this part of Cretaceous time. 



The Tertiary rests in this region unconformably on the Pierre, but 

 elsewhere the Fox Hills formation of sandstone and shale, 250 to 500 

 feet thick in the Black Hills, overlies the Pierre shale, and above the 

 Fox Hills come the Laramie and associated formations, several thousand 

 feet in thickness and containing unconformities. These sediments, how- 

 ever, were probably more rapidly accumulated. They are the record of 

 the Laramie revolution, marked by the upthrust of mountain ranges and 

 the downwarping of associated l)asins. The Dakota sandstone lies at the 

 base of the Cretaceous, below the shale series, and varies from nothing to 



''^ G. K. Gilbert : Sedimentary measurement of Cretaceous time. Jour. Geology, vol. 

 iii, 1895, pp. 121-127. 



