[263]

We might expect, supposing for the moment each formation to represent an equal duration of time, to be lost & disappear by (1) denudation, 2d metamorphosis 3d concealment by [illeg] surface. In well explored countries, owing to the generality of upheaval & aqueous erosion rare. According to this Silurian system ought to be most rare, & the Primordial Faunæ found only in few places. But this is not the case for extension of Silurian very great, has been thought (see Boué) equal to [illeg] of world. Certainly Silurian from some unknown cause much thicker than average, wh? cd explain some of extension; but why much thicker. Was the world quieter then?

N.B. Be sure consult Lyell Editions of Elements soon to appear after this present Nov/54/ for he has discussed this question. Murchison very strong for beginning of life. see his Silurian.

(Geolog)