Page:A Treatise on Geology, volume 1.djvu/282

 266 {| style="font-size:85%;line-height:140%;border-style:none;text-align:left;" cellspacing="0" cellpadding="2"
 * || $$\left. \begin{matrix}\ \ \end{matrix} \right\}$$ || 7390
 * }
 * }

But this last general average of the number of tertiary species now living, is composed of many very different ratios, by the study of which M. Deshayes has been led to class the tertiary formations upon a new principle. He assumes, as a general truth, that those tertiary deposits which contain the greatest proportion of existing species are of the most recent date; and on the contrary, that those in which the ratio of existing species is smallest are the oldest. Applying this principle to the most important localities of tertiary strata, and grouping together those which have the greatest agreements in ratio of living species, he arrives at the following series of three terms for the whole mass of tertiary strata.