882 .T. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



Another line of control is given by the recurrences of crust movements 

 which separate the periods. These pulse through earth history-, appar- 

 ently with some degree of regularity, and constitute the diastrophic basis 

 for the division of the eras into periods. 



The application of this principle may be seen in the Lower Paleozoic. 

 From the basis of stratigraphy, the Cambrian and Ordovician are con- 

 cluded, on the whole, to have been times characterized by widely flooded 

 continents. The lands were restricted in area and low in relief, yet wide 

 and thick limestone formations accumulated, indicating the passage of 

 geologically long intervals of time. A better appreciation of the extreme 

 length of these two periods is given by a close study of the diastrophic 

 record. Schuchert and Ulrich, tracing out disconformities as revealing 

 the coming and going of the epeiric seas, and delimiting the periods on 

 this basis, have found that both Ordovician and Cambrian consist of sev- 

 eral periods. On this basis Schuchert has recently subdivided the Ordo- 

 vician into four new periods and the Cambrian into three.^^^ This ex- 

 plains the unusual length, as given by stratigraphic evaluation. 



Thus, adjusting the evidence regarding land area, relief, rate of depo- 

 sition of sandstone, shale, and limestone, valuation of disconformities and 

 unconformities, and fitting these to the ages determined by uranium min- 

 erals, there may be constructed a geologic chronology. This evidence is 

 brought together in the following time table, the ratios for the periods 

 being obtained from a series of papers by various geologists in which these 

 have been individually estimated. A discussion of these ratios is next in 

 order. 



Dana, seeking the maximum thicknesses, taking each foot of limestone 

 to be the equivalent of five feet of sandstone and shale, and assuming a 

 uniform rate of accumulation throughout geologic time, obtained certain 

 ratios for the successive periods. H. S. Williams, in 1893, taking into 

 consideration the newer knowledge regarding the magnitude of Cambrian 

 and Ordovician sedimentation, slightly modified these ratios,^^^ his results 

 being given in the following table, page 884. Walcott made at this time 

 a careful study of the Paleozoic deposits of the Cordilleran sea and ob- 

 tained a somewhat dift'erent set of figures,^ ^^ the basis for which has been 

 discussed in another section of this article. He did not attempt to give 

 an estimate for each period, but dealt with the eras only. He considered 



13S c. Schuchert : Correlation and chronology ou the basis of paleogeography. Bull. 

 Geol. Soc. Am., vol. 27, 1916, p. 496. 



139 H. S. Williams : The elements of the geological time scale. Jour. Geo!., vol. i, 1893, 

 pp. 283-295. 



"0 C. D. Walcott : Geologic time as indicated by the sedimentary rocks of North Amer- 

 ica. Jour. Geol., vol. i, 1893, pp. 639-676. 



