Origin of Species Variorum

are good to show how slowly the mass must have been heaped together. ↑ Professor Ramsay has given me the maximum thickness, from actual measurement, in a few cases from estimate, of each formation in different parts of Great Britain; and this is the result:— Feet Palæozoic strata (not including igneous beds) .. 57,154 Secondary strata .. .. .. .. .. .. .. 13,190 Tertiary strata .. .. .. .. .. .. .. 2,240 — making altogether 72,584 feet; that is, very nearly thirteen and three-quarters British miles. Some of the formations, which are represented in England by thin beds, are thousands of feet in thickness on the Continent. Moreover, between each successive formation, we have, in the opinion of most geologists, enormously long blank periods. So that the lofty pile of sedimentary rocks in Britain gives but an inadequate idea of the time which has elapsed during their accumulation. .. .. .. .. .. .. .. The consideration of these various facts impresses the mind almost in the same manner as does the vain endeavour to grapple with the idea of eternity. ↑

Nevertheless this impression is partly false. Mr. Croll, in a most interesting paper, remarks that we do not err "in forming too great a conception of the length of geological periods," but in estimating them by years. When geologists look at large and complicated phenomena, and then at the figures representing several million years, the two produce a totally different effect on the mind, and the figures are at once pronounced to be too small. But in regard to denudation, Mr. Croll shows, by calculating the known amount of sediment annually brought down by certain rivers, relatively to the areas of drainage, that 1000 feet of rock, disintegrated through subaerial agencies, would thus be removed from the mean level of the whole area in the