s the base of the great central
plains of England, and is surmounted by the oliferous marls and red
arenaceous beds which pass under the succession of great oolitic
terraces that stretch across England from the coasts of Dorsetshire to
the north-eastern coast of Yorkshire. It marks the commencement of an
important era, being the strata in which land animals are first found.
The _Oolte System_ which follows marks the beginning of mammalia, and in
some of its beds in Buckinghamshire are found the exuviae of tropical
trees. Near Weymouth, in the well-known dirt beds, are found trees with
their silicified trunks growing up in the position of nature, and their
roots embedded in the soil on which they grew.

Next we have the chalk or _Cretaceous Formation_, that makes such a
conspicuous figure in England. The celebrated cliffs of Dover are of
this era. It forms a stripe from Yorkshire to Kent, and is found in
France, Germany, Russia, and in North America. The English chalk beds
are 1,200 feet thick, showing the considerable depth of the ocean in
which they were formed. Their origin has been a questionable topic; they
were thought to be formed from the detritus of coral reefs, but
Professor EHRENBERG has recently announced, as the result of his
microscopical researches, that chalk is composed partly of inorganic
particles and partly of shells of inconceivable minuteness, a cubic inch
of the substance containing about ten millions of them.

In the hollows of the chalk-beds have been formed series of
strata--clay, limestone, marl alternating--to which the name of the
_Tertiary System_ has been given. It is irregularly distributed over
vast surfaces of all our continents, and must be considered as the beds
of estuaries left at the conclusion of the cretaceous period. London and
Paris rest on basins of this formation, and another such basin extends
from near Winchester under Southampton, and reappears in the Isle of
Wight.

We hasten upward to the _Diluvial System_, which brings us near to the
present surface. To this era is referred the erratic blocks, or gigantic
boulder stones, which have been driven by floods across our continents,
or drifted in icebergs over valleys, and perched sometimes on mountain
tops. To it also must be referred the _till_ of Scotland and the great
brown clay of England, and our vast beds of gravel and superficial
rubbish, connected with the deluvium in the history of _ossiferous
caverns_, of which th