the thick deposits which have been
exposed to erosion, he comes upon one of those breaks in the
succession, or encounters what is called an unconformity, as when
horizontal strata lie against those which are tilted. In many cases he
may observe that at this time there was a great interval unrepresented
by deposits at the place where his observations are made, yet a great
lapse of time is indicated by the fact that a large amount of erosion
took place in the interval between the two sets of beds.

Putting together the bits of record, and assuming that the rate of
erosion accomplished by the agents which operate on the land has
always been about the same, the geologist comes to the conclusion that
the section of the rocks from the present day to the lowest strata of
the Laurentian represents in the time required for their formation not
less than a hundred million years; more likely twice that duration. To
this argument objection is made by some naturalists that the agents of
erosion may have been more active in the past than they are at
present. They suggest that the rainfall may have been much greater or
the tides higher than they now are. Granting all that can be claimed
on this score, we note the fact that the rate of erosion evidently
does not increase in anything like a proportionate way with the
amount of rainfall. Where a country is protected by its natural
coating of vegetation, the rain is delivered to the streams without
making any considerable assault upon the surface of the earth, however
large the fall may be. Moreover, the tides have little direct cutting
power; they can only remove detritus which other agents have brought
into a condition to be borne away. The direct cutting power of the
tidal movement does not seem to be much greater in the Bay of Fundy,
where the maximum height of the waves amounts to fifty feet, than on
the southern coast of Massachusetts, where the range is not more than
five. So far as the observer can judge, the climatal conditions and
the other influences which affect the wear of rocks have not greatly
varied in the past from what they are at the present day. Now and then
there have been periods of excessive erosion; again, ages in which
large fields were in the conditions of exceeding drought. It is,
however, a fair presumption that these periods in a way balance each
other, and that the average state was much like that which we find at
present.

If after studying the erosive phe