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continuous. There were series of successive faunas or even sub-faunas, in which were expressed the general features of the evolution of life on the globe. The species preserved and known present but a very imperfect representation of the species that were living; but of those preserved in one formation there are generally found in the succeeding formations representatives of the same or closely allied genera; so that for the kinds of organisms whose remains are best preserved the record is fairly continuous for the grander rock-systems in terms of the generic, and in some cases of the specific characters.

While the conditions of deposition for a particular region remained relatively constant and uniform, the strata were accumulated in successive beds one upon another, and then the thickness of the deposits of the same kind, with proportionate thickness for deposits of different kinds, constitute a scale of definite time value; a foot of deposit representing a period of time, and the relative time-separation for two faunas would be represented by the thickness of the strata between them. It was on this principle that the time-ratios of Dana were estimated. The maximum thickness of the known strata of each geological system was taken. The limestones were assumed to represent five times the time-value that is represented by the other sedimentary deposits per foot; or in other words, every foot of limestone was estimated as equivalent to five feet of other sedimentary deposits in making up the time-ratios. On this principle Dana estimated the time-ratio for the several geological periods to be as follows: