xliv INTRODUCTION. 
collecting fossils, many of which are now described in this volume. I believed his 
sections, so far as I saw them, to be true to nature; and I never suspected (nor had he 
then suspected) any discordancy, or break of continuity, amongst his typical rocks, 
from the Upper Ludlow down to the Llandeilo group. I adopted all his groups I might 
say with implicit faith; never dreaming of a chance (during a rapid visit) of correcting those 
elaborate sections on which he had bestowed so much successful labour. In one respect 
our examination of the sections was (as I now think strangely and almost wnaccountably) 
defective. We never examined, or discussed together, the Silurian base-line, in the country 
south of Welsh Pool; and whatever be the merit or demerit of the base-line afterwards 
published in the Map of the “Silurian System,” belongs exclusively to my friend. As to 
this base-line, I neither gave, nor had I an opportunity of giving, any opinion either good or 
bad. One thing however was certain, that my friend placed all the groups of South Wales, 
which I have called Upper Cambrian, below ali his Silurian groups; and if this were a 
mistake, I assuredly had no participation in it; nor had I the means, by any suggestion of 
my own, of leading him into it. That it was a great mistake is now certain; and it arose, 
beyond question, from a want of knowledge of the May Hill group, and the generally 
discordant relations of the Cambrian to the Silurian groups—a fact at that time unobserved 
in South Wales, and not even suspected by the author of the “Silurian System.” 
When we afterwards extended our joint examination of the natural sections to the 
district North of Welsh Pool, we had then reached a country (east of the Berwyns) with which 
I was previously acquainted; and of which I had commenced a geological map, and made some 
detailed field sections in 1832. My friend now made use of, and interpreted, some of my 
field sections of 1832. On his interpretation of them, the shelly sandstones of Meifod, (and 
consequently the calcareous slates of Glyn Ceiriog) became the equivalents of the Caradoc 
sandstone; and another calcareous band became the equivalent of the Llandeilo flag*. 
On the principles in which the “Silurian System” was worked out, this interpretation, 
so far as it regarded the Caradoc sandstone, was unquestionably correct; for the Meifod 
beds, as well as the calcareous slates of Glyn Ceiriog, are immediately under the Wenlock 
shale, without any apparent discordancy of position; and in Glyn Ceiriog the lower group 
actually seems to pass into the overlying Wenlock beds. Yet this appearance is deceptive: 
unless, indeed, we start the hypothesis, that the lower Wenlock beds, to the north of Glyn 
Ceiriog, are the actual equivalents of the May Hill sandstone on the western side of the 
Berwyn chain. In point of fact these sections, apparently so clear, are very deceptive; and 
they present those very difficulties which long misled geologists, and threw the lower groups 
of the “Silurian System” entirely out of their true geological place. 
Here, then, was the commencement of a difficulty, which, after 1842, led to a diver- 
gency of opinion and a controversy affecting both classification and nomenclature. Could 
the groups of Meifod, or Glyn Ceiriog, be so far separated from the group of calcareous 
* See the Sections of the “ Silurian System,” Pl. 32, fig. 9. 
