Many ingenious calculations have been made to approximate the dates
of certain geological events; but these, it must be confessed, are
more amusing than instructive. For example, so many inches of silt are
yearly laid down in the delta of the Mississippi--how many centuries
will it have taken to accumulate a thickness of 30, 60, or 100 feet?
Again, the ledges of Niagara are wasting at the rate of so many feet
per century--how many years must the river have taken to cut its way
back from Queenstown to the present Falls? Again, lavas and melted
basalts cool, according to the size of the mass, at the rate of so many
degrees in a given time--how many millions of years must have elapsed,
supposing an original igneous condition of the earth, before its crust
had attained a state of solidity? or further, before its surface had
cooled down to the present mean temperature? For these and similar
computations, the student will at once perceive we want the necessary
uniformity of factor; and until we can bring elements of calculation as
exact as those of astronomy to bear on geological chronology, it will
be better to regard our “eras” and “epochs” and “systems” as so many
terms, indefinite in their duration, but sufficient for the magnitude
of the operations embraced within their limits.--_Advanced Textbook of
Geology, by David Page, F.G.S._
M. Rozet, in 1841, called attention to the fact, that the causes which
have produced irregularities in the structure of the globe have not yet
ceased to act, as is proved by earthquakes, volcanic eruptions, slow
and continuous movements of the crust of the earth in certain regions,
&c. We may, therefore, yet see repeated the great catastrophes which
the surface of the earth has undergone anteriorly to the historical
period.
At the meeting of the British Association in 1855, Mr. Hopkins excited
much controversy by his startling speculation--that 9000 years ago
the site on which London now stands was in the torrid zone; and that,
according to perpetual changes in progress, the whole of England would
in time arrive within the Arctic circle.