294 THE JOURNAL OF GEOLOGY. 



the rate of accumulation of muds, sands and pebble beds, and of 

 the formation of limestones, in relation to each other and under 

 varying conditions, and the detection of the marks in the strata 

 recording the conditions incident to the varying rates of accum- 

 ulation. Until the evidence is fuller the time -ratios of Dana may 

 be adopted as expressing approximate values for the various 

 geological ages. 



In all these studies in which the geological time -scale is 

 applied to the evolution of the earth and its inhabitants, the time 

 concerned is not human chronology but is what may be called 

 geochronology. For this purpose we need a standard time -unit 

 or geochrojie. The geochrone applied in Dana's time -ratios 

 appears to be 8,000 feet of sedimentary deposits, as in the Pots- 

 dam, (7000 feet sediments and 200 limestone). Something more 

 definite is needed and one in which the equivalents in different 

 kinds of deposit and in different regions can be studied and 

 compared with some approach to accuracy. The Eocene period, 

 as expressed in the gulf states on both sides of the Mississippi 

 river, might be selected as a convenient and practicable standard 

 for this purpose. Humphrey and Abbot's elaborate studies of 

 the Mississippi river furnish minute data for comparison with 

 recent conditions. There are 3,000 feet of marine beds referred 

 to the Eocene in southern Europe. The Eocene or early Terti- 

 ary fresh -water beds reach a thickness of at least 10,000 feet. The 

 Tertiary beds in Liguria are estimated to reach the thickness of 

 23,600 feet. If for the present we assume the Eocene geochrone 

 to be equivalent to the maximum deposit of 3,000 feet of f rag- 

 mental sediment on the edge of the continent, using Dana's 

 estimates of time -ratios with some modifications, and adopting 

 the term Eocene as the American students of marine Eocene 

 apply it, the following standard time -scale for geochronology is 

 constructed. The geochrone in this scale is the period represent- 

 ed by the Eocene, as understood in North America to include the 

 marine deposits and their faunas, from the close of the Cretaceous 

 to the top of the Vicksburg or white limestone of Smith and John- 

 ston, 1,700 feet of which are seen in Alabama. In England it 



