. Murchison
contends theoretically for this relation between lithological character
and date. For on the page from which we have just quoted (_Siluria_,
p. 387), he says, that "whilst the soft Lower Silurian clays and sands
of St. Petersburg have their equivalents in the hard schists and quartz
rocks with gold veins in the heart of the Ural mountains, the equally
soft red and green Devonian marls of the Valdai Hills are represented on
the western flank of that chain by hard, contorted, and fractured
limestones." But these, and other such admissions, seem to go for
little. While himself asserting that the Potsdam-sandstone of North
America, the Lingula-flags of England, and the alum-slates of
Scandinavia are of the same period--while fully aware that among the
Silurian formations of Wales, there are oolitic strata like those of
secondary age; yet his reasoning is more or less coloured by the
assumption, that formations of like qualities probably belong to the
same era. Is it not manifest, then, that the exploded hypothesis of
Werner continues to influence geological speculation?

"But," it will perhaps be said, "though individual strata are not
continuous over large areas, yet systems of strata are. Though within a
few miles the same bed gradually passes from clay into sand, or thins
out and disappears, yet the group of strata to which it belongs does not
do so; but maintains in remote regions the same relations to other
groups."

This is the generally-current belief. On this assumption the received
geological classifications appear to be framed. The Silurian system, the
Devonian system, the Carboniferous system, etc., are set down in our
books as groups of formations which everywhere succeed each other in a
given order; and are severally everywhere of the same age. Though it may
not be asserted that these successive systems are universal; yet it
seems to be tacitly assumed that they are. In North and South America,
in Asia, in Australia, sets of strata are assimilated to one or other of
these groups; and their possession of certain mineral characters and a
certain order of superposition are among the reasons assigned for so
assimilating them. Though, probably, no competent geologist would
contend that the European classification of strata is applicable to the
globe as a whole; yet most, if not all geologists, write as though it
were. Among readers of works on Geology, nine out of ten carry away the
impression that the