boring. In mountainous districts, however, there are occasions when the
hade is to the up-throw, and this kind of fault is known as an _inverted
fault_.

Lines of faults extend sometimes for hundreds of miles. The great Pennine
Fault of England is 130 miles long, and others extend for much greater
distances. The surfaces on both sides of a fault are often smooth and
highly polished by the movement which has taken place in the strata. They
then show the phenomenon known as _slicken-sides_. Many faults have
become filled with crystalline minerals in the form of veins of ore,
deposited by infiltrating waters percolating through the natural
fissures.

In considering the formation and structure of the better-known
coal-bearing beds of the carboniferous age, we must not lose sight of the
fact that important beds of coal also occur in strata of much more recent
date. There are important coal-beds in India of Permian age. There are
coal-beds of Liassic age in South Hungary and in Texas, and of Jurassic
age in Virginia, as well as at Brora in Sutherlandshire; there are coals
of Cretaceous age in Moravia, and valuable Miocene Tertiary coals in
Hungary and the Austrian Alps.

Again, older than the true carboniferous age, are the Silurian
anthracites of Co. Cavan, and certain Norwegian coals, whilst in New
South Wales we are confronted with an assemblage of coal-bearing strata
which extend apparently from the Devonian into Mesozoic times.

Still, the age we have considered more closely has an unrivalled right to
the title, coal appearing there not merely as an occasional bed, but as a
marked characteristic of the formation.