sea. It is rather surprising to find these before any vegetable
forms, considering that vegetables appear to us as forming the
necessary first link in the chain of nutrition; but it is probable
that there were sea plants, and also some simpler forms of animal
life, before this period, although of too slight a substance to leave
any fossil trace of their existence.

The exact point in the ascending stratified series at which the first
traces of organic life are to be found is not clearly determined.
Dr. M'Culloch states that he found fossil orthocerata (a kind of
shell-fish) so early as the gneiss tract of Loch Eribol, in
Sutherland; but Messrs. Sedgwick and Murchison, on a subsequent
search, could not verify the discovery. It has also been stated,
that the gneiss and mica tract of Bohemia contains some seams of
grawacke, in which are organic remains; but British geologists have
not as yet attached much importance to this statement. We have to
look a little higher in the series for indubitable traces of organic
life.

Above the gneiss and mica slate system, or group of strata, is the
Clay Slate and Grawacke Slate System; that is to say, it is higher in
the ORDER OF SUPRAPOSITION, though very often it rests immediately on
the primitive granite. The sub-groups of this system are in the
following succession upwards:- 1, hornblende slate; 2, chiastolite
slate; 3, clay slate; 4, Snowdon rocks, (grawacke and conglomerates;)
5, Bala limestone; 6, Plynlymmon rocks, (grawacke and grawacke
slates, with beds of conglomerates.) This system is largely
developed in the west and north of England, and it has been well
examined, partly because some of the slate beds are extensively
quarried for domestic purposes. If we overlook the dubious