XX PREFACE. 
friend’s excursion along the strike of the Bala Limestone, and whether he had seen reason to 
make any change of position among his typical groups. He replied that he had followed the 
Limestone as far as the South-East flank of Cader Idris; and that he had no change to 
make in the position and relations of his fundamental groups (the Caradoc and Llandeilo) ; 
in short, that after careful re-examination of his Sections and Classifications, he had no 
mistake to correct. 
I had nothing to oppose to this, except my great surprise; and after reconsidering, for a 
while, my own Sections, I informed him, at the time of our discussion, that I could think 
of no place for his lower Silurian groups, unless they had disappeared among the rocks at 
the northern end of Berwyns, along the line of unconformable junction; either by an overlap 
of the upper or true Silurian rocks of Denbighshire, or, by some mistake of mine, in appre- 
ciating the calcareous and fossil-bearing groups, I had seen along that line: and to illustrate 
my view, I drew a rough outline sketch to shew how it was possible to interpolate, at the 
Northern end of the Berwyn and Glyn Ceiriog range, some beds, which might be higher than 
the Bala group, and on the parallel of the Caradoc sandstone. But I did this not with 
any view that my rough sketch should be published: for it was but an artifice to escape 
from a very great difficulty. It was in itself improbable, and it was based upon the asswmp- 
tion that the Lower Silurian Sections were true. Assuming this as a fact, I suggested, as a 
mere hypothesis, that the two groups, Caradoc and Llandeilo, might be concealed, or perhaps 
obscurely represented along the great line of discordant junction, which separated my Cam- 
brian from his true Silurian rocks, in Denbighshire. 
Such is the history of that Section which appears with my name affixed to it, at the 
North-East end of the great map of the “Silurian System.” The author of that map had no 
authority from me to publish the hypothetical sketch. It may appear strange that I should 
think it worth while thus to dwell upon a minute point which is now seldom seen or thought 
of; but it was, in fact, the very pivot on which my dispute with Sir R. I. Murchison turned. 
After we parted at the rich fossil-quarry near Bala, we never had one single syllable of 
correspondence respecting the older Paleozoic rocks of North Wales; nor did I again explore 
a single quarry of those rocks till full eight years had passed away. But, as before stated, the 
great work, the “Silurian System,” appeared in the early summer of 1839; and when I saw 
my hypothetical Section entered upon the map, with my name affixed to it, I thought I had 
some right to be offended at this liberty: for the author had invited me to colour that 
part of his map which was east of the Berwyn chain; and this I refused to do, on the 
ground of my incompetency, at that time, to colour the country correctly; and I should have 
given unquestionably a like refusal, if he had alluded to the sketch above described. My 
mistake was in believing, on the authority of their author, that the lower Silurian Sections 
