limestone, marbles and veins of the ores of lead, zinc, and antimony, and
occasionally silver.

[Illustration: FIG. 18.--Sigillarian trunks in current-bedded sandstone.
St Etienne.]

As the most apparent of the rocks of the system are sandstone, shale,
limestone, and coal, it will be necessary to consider how these were
deposited in the waters of the carboniferous ages, and this we can best
do by considering the laws under which strata of a similar nature are now
being deposited as sedimentary beds.

A great proportion consists of sandstone. Now sandstone is the result of
sand which has been deposited in large quantities, having become
indurated or hardened by various processes brought to bear upon it. It is
necessary, therefore, first to ascertain whence came the sand, and
whether there are any peculiarities in its method of deposition which
will explain its stratification. It will be noticed at once that it bears
a considerable amount of evidence of what is called "current-bedding,"
that is to say, that the strata, instead of being regularly deposited,
exhibit series of wedge-shaped masses, which are constantly thinning out.

Sand and quartz are of the same chemical composition, and in all
probability the sand of which every sandstone in existence is composed,
appeared on this earth in its first solid form in the shape of quartz.
Now quartz is a comparatively heavy mineral, so also, therefore, will
sand be. It is also very hard, and in these two respects it differs
entirely from another product of sedimentary deposition, namely, mud or
clay, with which we shall have presently to deal when coming to the
shales. Since quartz is a hard mineral it necessarily follows that it