n the upper Silurian beds, and abounding in vast
variety of strange forms in the old Red Sandstone, but gradually
disappearing from the waters of the world, till their only
representatives, as far as known, are the Lepidostei, or "Bony
Pikes," of North America; the Polypteri of the Nile and Senegal; the
Lepidosirens of the African lakes and Western rivers; the Ceratodus
or Barramundi of Queensland (the two latter of which approach
Amphibians), and one or two more fantastic forms, either rudimentary
or degraded, which have lasted on here and there in isolated stations
through long ages, comparatively unchanged while all the world is
changed around them, and their own kindred, buried like the fossil
Ceratodus of the Trias beneath thousands of feet of ancient rock,
among creatures the likes whereof are not to be found now on earth.
And these are but two examples out of hundreds of the vast changes
which have taken place in the animal life of the globe, between the
laying down of the Cambrian slates and the present time.

Surely--and it is to this conclusion I have been tending throughout a
seemingly wandering paragraph--surely there has been time enough
during all those ages for clay to change into slate.

And how were they changed?

I think I cannot teach my readers this more simply than by asking
them first to buy Sheet No. LXXVIII. S.E. (Bangor) of the Snowdon
district of the Government Geological Survey, which may be ordered at
any good stationer's, price 3s.; and study it with me. He will see
down the right-hand margin interpretations of the different colours
which mark the different beds, beginning with the youngest (alluvium)
atop, and going down through Carboniferous Limestone and Sandstone,
Upper Silurian, Lower Silurian, Cambrian, and below them certain
rocks marked of different shades of red, which signify rocks either
altered by heat, or poured out of old volcanic vents. He will next
see that the map is covered with a labyrinth of red patches and
curved lines, signifying the outcrop or appearance at the surface of
these volcanic beds. They lie at every conceivable slope; and the
hills and valleys have been scooped out by rain and ice into every
conceivable slope likewise. Wherefore we see, here a broad patch of
red, where the back of a sheet of Lava, Porphyry, Greenstone, or what
not is exposed; there a narrow line curving often with the curve of
the hill-side, where on