ages when one kind seems to have been the _chief_ kind--an age of
limestone, or an age of chalk. But even then there were doubtless more
rock-buildings going on, though not to so great an extent. On the
other hand, there may have been ages during which no limestone was
made, or no chalk, or no clay. As a general rule, however, the various
sorts of rock-building have probably gone on together. This was not so
well understood by early geologists as it is now.

The difficulty is often great of disentangling the different strata,
and saying which was earlier and which later formed.

Still, by close and careful study of the rocks which compose the
earth's crust, a certain kind of order is found to exist, more or less
followed out in all parts of the world. _When_ each layer was formed
in England or in America, the geologist cannot possibly say. He can,
however, assert, in either place, that a certain mass of rock was
formed before a certain other mass in that same place, even though
the two may seem to lie side by side; for he knows that they were so
placed only by upheaval, and that once upon a time the one lay beneath
the other.

The geologist can go further. He can often declare that a certain mass
of rock in America and a certain mass of rock in England, quite
different in kind, were probably built up at about the same time. How
long ago that time was he would be rash to attempt to say; but that
the two belong to the same age he has good reason for supposing.

We find rocks piled upon rocks in a certain order, so that we may
generally be pretty confident that the lower rocks were first made,
and the upper rocks the latest built. Further than this, we find in