me time, and ceasing to subside at
the same time--a coincidence which alone could produce equivalent groups
of strata. Subsidences in different places begin and end with utter
irregularity; and hence the groups of strata thrown down in them can but
rarely correspond. Measured against each other in time, their limits
must disagree. On turning to the evidence, we find that it daily tends
more and more to justify these _a priori_ positions. Take, as an
example, the Old Red Sandstone system. In the north of England this is
represented by a single stratum of conglomerate. In Herefordshire,
Worcestershire, and Shropshire, it expands into a series of strata from
eight to ten thousand feet thick, made up of conglomerates, red, green,
and white sandstones, red, green, and spotted marls, and concretionary
limestones. To the south-west, as between Caermarthen and Pembroke,
these Old Red Sandstone strata exhibit considerable lithological
changes; on the other side of the Bristol Channel, they display further
changes in mineral characters; while in South Devon and Cornwall, the
equivalent strata, consisting chiefly of slates, schists, and
limestones, are so wholly different, that they were for a long time
classed as Silurian. When we thus see that in certain directions the
whole group of deposits thins out, and that its mineral characters
change within moderate distances; does it not become clear that the
whole group of deposits was a local one? And when we find, in other
regions, formations analogous to these Old Red Sandstone or Devonian
formations, is it certain--is it even probable--that they severally
began and ended at the same time with them? Should it not require
overwhelming evidence to make us believe as much?

Yet so strongly is geological speculation swayed by the tendency to
regard the phenomena as general instead of local, that even those most
on their guard against it seem unable to escape its influence. At page
158 of his _Principles of Geology_, Sir Charles Lyell says:--

"A group of red marl and red sandstone, containing salt and gypsum,
being interposed in England between the Lias and the Coal, all
other red marls and sandstones, associated some of them with salt,
and others with gypsum, and occurring not only in different parts
of Europe, but in North America, Peru, India, the salt deserts of
Asia, those of Africa--in a word, in every quarter of the globe,
were referred to one