INTRODUCTION. Ixi 
observed succession of such groups we never could have dreamt of a natural succession of 
paleontological types; or used such types in proof of a long succession of epochs recorded 
in the ancient chronicles of the earth. These principles have been happily applied in elimi- 
nating and classifying the geological deposits of all ages; and they have never (I repeat) 
been abandoned with impunity and without loss. And if it has been found needful to adhere 
to these principles among the acknowledged British groups, from the upper Palzozoic to 
the newest Tertiary,—and if it has been found, from past experience, that the exact determi- 
nation of physical groups has, in every age of Geology, been the first step in advance: still 
more are we bound to these first principles while we are dealing with the lowest division of the 
whole Paleozoic System; because, within the limits of that division, fossil evidence gradually 
disappears, and physical evidence becomes, at length, not only our first, but our only guide. 
When I entered North Wales in 1831, it was, geologically, little more than a terra 
incognita. Honestly, and with no small labour, I worked my way through this unknown 
land. I was guided by the physical groups which marked a true physical succession; and 
in classifying and naming the successive groups I never once deserted the great first prin- 
ciple of double evidence, so far as it admitted of application. The final result appears in 
the above Tabular View. 
The “Silurian System” was admirably worked out, on the same acknowledged prin- 
ciple, as far as its base—the May Hill Sandstone; and (without, for the present, making any 
count of the May Hill Sandstone) every downward step in the “Silurian System” professed 
to be made on the same double evidence of sections and fossils, till the whole series ended 
in the Llandeilo Flag, which had its supposed base resting upon the highest of the great 
collective Cambrian groups of the Tabular View. In the nomenclature of the “System” 
the true principle of classification was acknowledged and proclaimed ex cathedrd by the 
author; and it was incontestably at the same time assumed, that the physical groups, from 
the Upper Ludlow rock down to the Llandeilo Flag and Upper Cambrian rocks, were 
arranged in the very order of nature*. 
* This was assumed as the true order in the typical section engraved on the original map of the Silurian System, 
and in all the other sections of the work that had any bearing on the point; and it was assumed as the true 
order (virtually the groundwork of the whole scheme of Silurian Classification and Nomenclature) in [Dr] Fitton’s 
Article in the Edinburgh Review (April, 1841) above referred to. At the time that Article was in preparation 
for the press the Author of the System seems still to have upheld the geological truth of his own sections. 
It would be a pitiful criticism upon a work of such great extent and importance (the publication of which 
(1839) formed an epoch in the history of Paleozoic geology) to say that its boundary lines were here and there 
a little divergent from a true geographical delineation. But it is not a small or pitiful criticism to prove that 
its base line was wrong in principle, and that the colours of the Silurian Map were erroneous in principle. For 
in the arrangement of the colours it was assumed (and for that assumption the author of the “Silurian System” was 
alone responsible) that the collective Upper Cambrian groups of South Wales (including all the undulating rocks 
between Cardigan Bay and the true Silurian terrace) were inferior to the base line of the Llandeilo Flag. The 
change made in the Silurian Map in 1843 was not an adjustment of a base line: it was a complete change of 
principle, which properly and naturally included a change of nomenclature. But the old nomenclature, which was 
erroneous because derived from erroneous sections, was kept entire; and the change was made in a wrong direction, 
by throwing the whole mistake upon the true geographical nomenclature, by sinking the name of Cambria, and 
by involving all Cambria in Siluria. A more direct offence against sound inductive logic and geographical propriety 
has never, perhaps, been committed in the history of geology. 
