extensive sylvan regions, it is thought that the vegetation, the rubbish
of decayed forests, was carried by rivers into estuaries, and there
accumulated into vast natural rafts, until it sank to the bottom, where
an overlayer of sand or mud would prepare it for becoming a stratum of
coal. Others conceive that the vegetation first went into the condition
of peat moss, that a sink in a level then exposed it to be overrun by
the sea and covered with a layer of sand or mud; that a subsequent
uprise made the mud dry land, and fitted it to bear a new forest, which
afterwards, like its predecessors, became a bed of peat--that, in short,
by repetitions of this process the alternate layers of coal, sand and
shell constituting the carboniferous group were formed.

The _Magnesian Limestone_ deposits succeed the carboniferous, and
sometimes pass into them by insensible gradations. In the south of
England they are represented by conglomerates, and partly composed of
the solid and more or less rounded fragments of the older strata. They
afford a proof of what geologists have often occasion to remark of the
long periods of time during which the ancient works of nature were
perfected; for the older rocks were solid as they are now, and their
organic remains petrified at the time these conglomerates were forming.

We can only briefly glance at the remaining chapters of geological
history. The _New Red Sandstone_ forms 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 exuviæ of tropical