INTRODUCTION. xiii 
of my joint work with Mr Salter, in 1842 and 1843, were also copied from my old note- 
book. Not, I believe, so much as a single important line, in any of my old sections in 
Caernarvonshire and Merionethshire, was touched during the two summers I spent in North 
Wales with Mr Salter*: and the Cambrian groups of the above Tabular View are (with the 
exception of two new names—Lingula jiag and Tremadoc slate—given to two previously 
acknowledged subgroups) literally copied from my field-notes of 1831 and 1832. I had 
no wish (after 1834) to publish any Cambrian details before the promised “Silurian System” 
had made its appearance; and I had no fear of intrusion into a field of such extent and 
great difficulty as the one in which I had been toiling both in Cumberland and Wales. 
Moreover, the upper and discordant groups (the Denbigh flagstone, &c.) of North Wales 
were obviously far less perfect than the corresponding groups in Siluria which, year by 
year, were brought under notice by my friend and fellow-labourer. At the time to which 
I allude (between 1834 and 1842) I did not care a straw about the priority of nomenclature 
among any of my upper groups (whether in Wales or the North of England); and I adopted, 
at once and without any reserve, all the Silurian names of the subgroups so far as they 
admitted of a plausible application; and had my friend ever proved that one of his lower 
Silurian groups fairly included any one of my upper Cambrian groups, I should have given 
that group up to him without a minute’s hesitation, and called it Silurian. 
There were, however, early difficulties, both physical and palzontological, in distinguishing 
the lower Silurian from the upper Cambrian groups, and in fixing their true geographical 
limits: and it was partly in the hopes of settling such points of doubt, that in 1834 I went 
during six weeks, under my friend’s personal guidance, to examine the order of succession, 
as established by himself in the typical Silurian country. Beginning, therefore, at Llandeilo, 
and ending the first part of our joint-work at Welsh Pool, we examined many of his best 
sections. Occasionally, while he was working out minute details, I spent some days in 
* Among several other sections exhibited by me in 1838 was one (copied from my note-book by Mr Lonsdale) from 
the Menai Straits to the Berwyn chain inclusive: (1) showing the position of the fossil trough (the equivalent of the 
Snowdonian trough) at the top of Moel Hebog; (2) all the anticlinal lines, and the fundamental position of Merioneth 
anticlinal; (3) the great ascending section from that anticlinal to the Bala limestone, and the groups superior to it, with its 
reappearance (with a reversed dip) beyond the Berwyn water-shed ; (4) the contemporaneous slates and porphyries which 
emerge from below the Bala limestone; after which the continuity of the section is arrested by enormous faults, and by 
an entire change of strike on the east side of the Berwyn chain. This section was afterwards exhibited on every occasion 
on which I described the Cambrian rocks. Another section took up the details of the preceding, and carried them forward 
to the northern end of the Berwyn chain near Corwen, where the series appeared to terminate. This section was perfectly 
true in its essential details. But how was it to be reconciled to the Lower Silurian Sections of Murchison? Here was an 
avowed difficulty; and the reconciliation was afterwards found to be impossible, not from any error in my sections exhi- 
bited in 1838, but simply because the “ Lower Silurian ” Sections of Murchison were upon a mistaken base. 
Let no one think, from the expressions in the text, that I undervalue the help I received from Mr Salter. His help 
was invaluable. Besides collecting fossils, we worked out many points of minute detail; and we mapped some large tracts 
of North Wales with an accuracy equal to that of the Government Survey. We worked out the sections east of the Berwyns 
and north of the Holyhead road far better than I had time to do in 1832; and he enabled me to put unequivocally in its 
right place, and to map as “ Upper Silurian” the sandstone (now May Hill sandstone) at the base of the Denbigh flag. 
{ To avoid wnnecessary periphrasis I have here and in other places used terms such as Cambrian and Silurian, even 
where I am alluding to periods when these terms had not been introduced. This anticipation can lead to no mistake. 
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