xlvi INTRODUCTION. 
The following day we parted, and we never afterwards, before the publication of the 
“Silurian System” in 1839, had any interchange of opinion respecting Cambria or Siluria. 
I went to re-examine the relations of the Denbigh flagstone, and the underlying sandstone, 
to the beds of the Bala group; and then, as before in 1852, I saw good evidence to prove that 
the upper and lower groups were discordant in position. But I was called away before I 
could return to any of the most critical sections at the contact of Cambrian and Silurian 
rocks, or attempt to settle any remaining doubt, whether the Meifod and Ceiriog beds were, 
or were not, to be separated from the Bala Group. 
In 1835 I was prevented, to my great sorrow, from revisiting the critical sections 
alluded to above; and after that year I was anxious that my friend should gather in his 
rich Silurian harvest before I again put my sickle into that of Cambria. Certainly he 
had fair play in bringing his work to maturity; for not many days after we parted in 1834, 
I left the Principality, and never again set my foot either in North Wales or Siluria 
before the summer of 1842. 
The question between us had been reduced (in 1834) to a very narrow compass. J/ the 
Silurian sections were right (and they were exhibited year by year and discussed again and 
again without a single opposing voice*)—then the Bala limestone (on my interpretation of 
the Upper Cambrian sections) must be at the very least five or six thousand feet below 
the Llandeilo flag. Jf on the other hand, the Glyn Ceitriog or Meifod beds were exactly, 
or nearly, on the parallel of the Bala limestone group, it then seemed inevitably to follow,— 
that a great error must have been committed in the “Lower Silurian” sections, and that 
the relations of the Caradoc and Llandeilo and Cambrian groups had been utterly mistaken 
by the author of the “Silurian System ;’—that, in short, the Llandeilo group (and the 
Caradoc group also, if the Meifod beds were to be called Caradoc) instead of being at 
the top, was near the bottom, of my Upper Cambrian group. 
I did not, in 1834, venture to draw a conclusion so fatal to all the typical “lower 
Silurian” sections, and to the integrity of the “System;” and I then thought it possible, 
that by help of enormous faults, and the obscurity of a discordant junction, we might in 
the end find in North Wales some representative of the Llandeilo flag above the Bala 
limestone group. This hypothesis (it was a mere hypothesis, not at all suggested by my 
own sections; but it was provisionally thrown out, on the supposed integrity of the Lower 
Silurian sections) was dissipated at once when I returned to North Wales in 1842, accom- 
panied by my friend Mr Salter. I knew the critical sections we had to examine on the line 
of junction between the Cambrian and Silurian rocks. They all told one story, and proved 
* See an article by Dr Fitton in the Edinburgh Review (Vol. Lxxu. p. 1, 1841) for an excellent synopsis of the 
“Silurian System.” There, and in every other published section, the descending order was Ludlow, Wenlock, Caradoc, 
Llandeilo, Cambrian. With the old slates of the Longmynd there was an obviously discordant junction; but in no other 
case was any obyious discordancy acknowledged or described. Thus the Author of the Silurian System, in a paper of 1836, 
described the slates of Caermarthenshire as the upper part of the “Cambrian System,” and added—that among them 
were beds of passage from the Cambrian to the Silurian System. Proceedings of the G'eol. Soc. Vol. 1. p. 229. This view 
was not contradicted, but was reaffirmed, on further evidence, when the “ Silurian System” appeared in 1839. 
