Our tests of geologic time are denudation and deposition. We are told "the present rate of denudation of a continent is known with _considerable accuracy_ from careful measurements of the quantity of solid matter carried down by rivers." [84] Now it is a considerable tax on our faith in science to believe that the _débris_ of the Mississippi can be so accurately gauged as to give anything like approximate value to the result of one foot of continental denudation in 6,000 years. We cannot of course suppose this to be the result of 6,000 years registered observations, but an inference from the observations of some comparatively insignificant period; and we have also to suppose that the very few rivers which have been observed form a sufficient basis for a conclusion as to all rivers. In fact, a more feebly supported generalization from more insufficient data it is hard to conceive. To speak of it as "an _approximation_ based on our knowledge of the time in which similar results on a smaller scale have been produced by existing natural laws within the historical period," [85] is a very inadequate qualification, especially when we have just been told that "here, at any rate, we are on comparatively certain ground, ... these are measurable facts which have been ascertained by competent observers." [86] Assuming this rate of denudation as certain, and also the estimate of the known sedimentary strata as 177,000 feet in depth, we are to conclude that the formation took 56,000,000 years. A mountain mass which ought to answer to certain fault 15,000 high, and therefore is presumed to have vanished by denudation, points to a term of 90,000,000 years as required for the process. [87]