the tract between Dumfries and
Moffat, in 1850, it occurred to me, that the dull reddish or purple
sandstone and schist to the north of the former town, which so resembled
the bottom rocks of Longmynd, Llanberis, and St. David's, would prove to
be of the same age;" and further on, he again insists upon the fact that
these strata "are absolutely of the same composition as the bottom rocks
of the Silurian region." On this unity of mineral character it is, that
this Scottish formation is concluded to be contemporaneous with the
lowest formations in Wales; for the scanty paleontological evidence
suffices for neither proof nor disproof. Now, had there been a
continuity of like strata in like order between Wales and Scotland,
there might have been little to criticize in this conclusion. But since
Sir R. Murchison himself admits, that in Westmoreland and Cumberland,
some members of the system "assume a lithological aspect different from
what they maintain in the Silurian and Welsh region," there seems no
reason to expect mineralogical continuity in Scotland. Obviously,
therefore, the assumption that these Scottish formations are of the same
age with the Longmynd of Shropshire, implies the latent belief that
certain mineral characters indicate certain eras. Far more striking
instances, however, of the influence of this latent belief remain to be
given. Not in such comparatively near districts as the Scottish lowlands
only, does Sir R. Murchison expect a repetition of the Longmynd strata;
but in the Rhenish provinces, certain "quartzose flagstones and grits,
like those of the Longmynd," are seemingly concluded to be of
contemporaneous origin, because of their likeness. "Quartzites in
roofing-slates with a greenish tinge that reminded us of the lower
slates of Cumberland and Westmoreland," are evidently suspected to be of
the same age. In Russia, he remarks that the carboniferous limestones
"are overlaid along the western edge of the Ural chain by sandstones and
grits, which occupy much the same place in the general series as the
millstone grit of England;" and in calling this group, as he does, the
"representative of the millstone grit," Sir R. Murchison clearly shows
that he thinks likeness of mineral composition some evidence of
equivalence in time, even at that great distance. Nay, on the flanks of
the Andes and in the United States, such similarities are looked for,
and considered as significant of certain ages. Not that Sir R