an that of Krakatoa; lava-streams more
extensive than that of Skaptar-Jokul, and earthquakes more powerful than
that which uplifted five or six hundred miles of the Pacific coast of
South America six or seven feet." [82] Now, seeing that all these
cataclysms have occurred within the brief limits of most recent time,
compared with which the period of pretended uniformity is almost an
eternity, what sort of presumption or probability is there that such
occurrences should have been confined to historical times; and is not
the presumption all the other way? Again, it is largely on the
supposition of this antecedently unlikely uniformity, that Mr. Laing
argues to the antiquity of life on earth; whereas Lyell's conclusion
warrants nothing of the kind, being simply: that present causes, "_given
sufficient time_," would produce the observed effects. [83]

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 _debris_ 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