line
marble, granular sandstones into firm masses, known as quartzites, and
clays into the harder form of slate. Where the changes go to the
extreme point, rocks originally distinctly bedded probably may be so
taken to pieces and made over that all traces of their stratification
may be destroyed, all fossils obliterated, and the stone transformed
into mica schist, or granite or other crystalline rock. It may be
injected into the overlying strata in the form of dikes, or it may be
blown forth into the air through volcanoes. Involved in
mountain-folding, after being more or less changed in the manner
described, the beds may become tangled together like the rumpled
leaves of a book, or even with the complexity of snarled thread. All
these changes of condition makes it difficult for the geologist to
unravel the succession of strata so that he may know the true order of
the rocks, and read from them the story of the successive geological
periods. This task, though incomplete, has by the labours of many
thousand men been so far advanced that we are now able to divide the
record into chapters, the divisions of the geologic ages, and to give
some account of the succession of events, organic and geographic,
which have occurred since life began to write its records.

EARTHQUAKES.

In ordinary experience we seem to behold the greater part of the earth
which meets our eyes as fixed in its position. A better understanding
shows us that nothing in this world is immovable. In the realm of the
inorganic world the atoms and molecules even in solid bodies have to
be conceived as endowed with ceaseless though ordered motions. Even
when matter is built into the solid rock, it is doubtful whether any
grain of it ever comes really to rest. Under the strains which arise
from the contraction of the earth's interior and the chemical changes
which the rocks undergo, each bit is subject to ever-changing
thrusts, which somewhat affect its position. If we in any way could
bring a grain of sand from any stratum under a microscope, so that we
could perceive its changes of place, we should probably find that it
was endlessly swaying this way and that, with reference to an ideally
fixed point, such as the centre of the earth. But even that centre,
whether of gravity or of figure, is probably never at rest.

Earth movements may be divided into two groups--those which arise from
the bodily shifting of matter, which conveys