eds, and
the soil upon it, is laid bare, the black layers of slate may be seen
gradually melting--if I may use the word--under the influence of rain
and frost, into a rich tenacious clay, which is now not black, like
its parent slate, but red, from the oxidation of the iron which it
contains.

But, granting this, how did the first change take place?

It must be allowed, at starting, that time enough has elapsed, and
events enough have happened, since our supposed mud began first to
become slate, to allow of many and strange transformations. For
these slates are found in the oldest beds of rocks, save one series,
in the known world; and it is notorious that the older and lower the
beds in which the slates are found, the better, that is, the more
perfectly elaborate, is the slate. The best slates of Snowdon--I
must confine myself to the district which I know personally--are
found in the so-called "Cambrian" beds. Below these beds but one
series of beds is as yet known in the world, called the "Laurentian."
They occur, to a thickness of some eighty thousand feet, in Labrador,
Canada, and the Adirondack mountains of New York: but their
representatives in Europe are, as far as is known only to be found in
the north-west highlands of Scotland, and in the island of Lewis,
which consists entirely of them. And it is to be remembered, as a
proof of their inconceivable antiquity, that they have been upheaved
and shifted long before the Cambrian rocks were laid down
"unconformably" on their worn and broken edges.

Above the "Cambrian" slates--whether the lower and older ones of
Penrhyn and Llanberris, which are the same--one slate mountain being
worked at both sides in two opposite valleys--or the upper and newer
slates of Tremadoc, lie other and newer slate-bearing beds of
inferior quality, and belonging to a yet newer world, the "Silurian."
To them belong the Llandeilo flags and slates of Wales, and the
Skiddaw slates of Cumberland, amid beds abounding in extinct fossil
forms. Fossil shells are found, it is true, in the upper Cambrian
beds. In the lower they have all but disappeared. Whether their
traces have been obliterated by heat and pressure, and chemical
action, during long ages; or whether, in these lower beds, we are
actually reaching that "Primordial Zone" conceived of by M. Barrande,
namely, rocks which existed before living things had begun to people
this planet, is a qu