ESTIMATES OF TIME 833 



several hundred feet in thickness. The 3,900 feet of shale may therefore 

 represent two-thirds or one-half of Cretaceous time, excluding the Lower 

 Cretaceous or Comanche as a separate period. This would give 30,000,000 

 to 40,000,000 years as the length of the Cretaceous ; but, as shown later, 

 this accords in order of magnitude with the estimates of the whole of the 

 Mesozoic era based on radioactivity. If the latter should prove to l)e 

 sound, it appears not improbable, therefore, that Gilbert has fortunately 

 detected running through the Cretaceous the beating of the precessional 

 rhythm. 



Professor Schuchert has told the writer that when Gilbert gave this 

 paper he and Dr. Stanton calculated that it would have taken some 6,000 

 years at this rate for a large Inoceramus to have become buried in sedi- 

 ment. As these large fossils are well preserved in many beds, showing 

 but little solution or worm boring, it seemed at the time a strong argu- 

 ment against such an extravagant estimate of the length of the Cre- 

 taceous. The answer, as indicated in the preceding parts of this paper, 

 is that the shells of living or recently dead mollusks were buried rapidly, 

 perhaps in a single culminating storm, by a blanket of wave-stirred mud. 

 During times of slow accumulation the successive generations of shells 

 would be completely destroyed by boring animals, by solution, and by 

 the recurrent wear of wave action. Thus it is characteristic of fossil if- 

 erous formations that the fossils occur in thin layers between much 

 thicker unfossiliferous beds. Within the layer the shells are commonly 

 well preserved, though often showing a disturbance by wave or current 

 action. The preservation of fossils is, then, following this view, generally 

 due to the recurrence of culminating storms at long intervals, which, 

 stirring the bottom to unusual depths, suddenly bury a layer of organic 

 debris beneath a protecting mantle of argillaceous or calcareous mud. 



Such an action seems necessary for the preservation of fossils, even 

 with the current estimates of geologic time. For example, let Cretaceous 

 time be taken as 3,000,000 or 4,000,000 years in length and the burial 

 of a large Inoceramus in the muds previously described would, with an 

 even rate of deposition, have required 600 years. This, when tested by 

 their state of preservation, seems to raise difficulties hardly less great than 

 a period of 6,000 years. Generally it would be only the fixed or dead 

 shells which would be entombed by mud blankets deposited by the ex- 

 ceptional storms in the stormy phase of a climatic cycle, but less fre- 

 quently the deposition of sediment would be so deep and rapid that even 

 the living members of the free bottom fauna would be smothered and 

 buried. As an illustration may be icited the slab of Devonian sandstone 



