Page:The Journal of geology (IA journalofgeology11893univ).pdf/308

Ward, in the fifth annual report of the United States Geological survey, has proposed to adjust these proportions as follows:

thus forming nine divisions of equal length.

Since Dana's estimate additions have been made to the known thickness of the Cambrian rocks of North America, which should lengthen the Cambrian ratio to 5 in the above table, and duplications of thickness due to confusion in regard to the Quebec group may reduce the Ordovician (Lower Silurian) to 5, and the Cretaceous ratio may be somewhat enlarged. The Tertiary estimate in Dana's ratios assumes the thickness to be of less (½) time-value because of the increased rate of deposition due to transportation of rivers. This and many other factors enter in to complicate the time-value of thickness of strata; but it must be granted that the thickness of the sediments is the prime factor in determining these time-values of the geological scale. However, the conditions of deposition, the fineness or coarseness of the clastic fragments, the abundance or rarity of supply of materials and other variable conditions must be taken into consideration in an accurate reduction of thickness of strata into length of time. Errors, also, whose value is almost impossible of estimation, arise from the intervals between strata, particularly those where unconformity exists.

After all these uncertainties are weighed the time-ratios formed on this general basis are of great importance in studying the history of organisms, and the value of accuracy in the time-scale is a sufficient reason for calling attention to the points in which greater