Ixxxvi INTRODUCTION. 
reous beds of this formation (the ‘Llandeilo flags’) as they lie to the west and north-west of the flags; and the same 
nodular limestones occur at Llangathen and Grongar Hills in similar positions, rising from beneath the younger strata, 
and graduating on their western flanks into the rotten slates and greywacke grits of the Inferior or Cambrian System, 
in which (in this district) all traces of fossils are lost.” 
In the next page (357) the Author describes a range of beds near the hamlet of Rhew-yr-adar, “as the best exam- 
ples in Caermarthenshire where this formation (Llandeilo flag) passes into strata of the Inferior System.” In the same 
paragraph, after describing certain sandy beds, which he supposes to be at the base of the Llandeilo flags; he adds, that 
there is “an imperceptible passage from these sandy beds into still more ancient strata void of fossils: viz. the black 
schists, which, in common with Professor Sedgwick, 1 consider the link connecting the Cambrian and Silurian Systems. 
Similar successions are observable near Llan-rhaidr in Denbighshire (p. 307), and in Pembrokeshire.” Again (p. 358), he 
considers that the fossiliferous beds, to the west of Noedd-Griig, “ occupy the base of the Llandeilo formation :” and in a 
previous page (352) he had stated that these strata of Noedd-Griig “ constituted the most interesting tract of Caermarthen- 
shire; since they exhibit a passage on the one side into the Upper Silurian Rocks, and on the other into the Upper Cambrian.” 
Again, he describes the “ Cambrian rocks” between Llandovery and Llampeter, and between St Clears and Newcastle Emlyn, 
without expressing any doubt as to the general order of superposition—any misgiving as to the fact that his Llandeilo 
group was superior to the rocks which he very properly had called Cambrian (p. 361). 
As a plain matter of fact (first, I believe, brought to light by the Government Survey), every section, aboye alluded 
to, was misinterpreted; the great groups were turned upside down; the Cambrian groups were not below, but above, the 
Llandeilo flags. I had no share, whatsoever, in the origination of this fatal mistake ; and (with the exception of Noedd-Griig) 
I never so much as saw (in 1834) any one of the critical sections alluded to above. I never gaye any opinion about the 
black schists and beds of passage (between the Cambrian and Silurian rocks of South Wales), or knew of their existence 
from any observations of my own. Nay, as a general rule, J did not believe in such beds of passage; but rather thought 
(on the analogy of North Wales) that the passage from Cambrian to Silurian rocks must be through the intervention 
of great dislocations and unconformable junctions. As for the errors of the Noedd-Griig sections, the supposed down- 
ward passage of tho beds into the Upper Cambrian rocks, and the supposition that the fossiliferous beds at the west side 
of the ridge “occupy the base of the Llandeilo formation,” I had no share in these original mistakes of interpretation. 
I did (in 1834) visit the summit of Noedd-Griig; but I saw no beds of passage, nor did I, on the day of that visit, 
accompany my friend to those western fossiliferous beds which he (erroneously) regarded as “the base of the Llandeilo 
formation.” 
For the succession of the groups near Llan-rhaidr, alluded to above, described in the Silurian System, p. 307, 
and delineated Plate XXXII. fig. 9, I am in part responsible. The S.S.E. end of the section is exclusively by Murchison ; 
the central part, from the vale of Meifod to the Tanat, was copied by him from my note-book of 1832. For the general 
position of the beds up to Craig-y-glyn, I am in part responsible; as we traversed that portion of the section together. 
For the general position and dip of the beds which rise into the eastern flank of the Berwyn mountains he is alone respon- 
sible. There is, in fact, an enormous down-cast fault (not represented in his section), a little beyond Craig-y-glyn; and 
the prevailing dip of the higher Berwyns (as I knew well from my own sections of 1832) is the very reverse of that which 
is given in the Plate. The beds in this part of the higher Berwyns have a westerly dip; which carries them, on the west 
side of the chain, under the May Hill Sandstone and the Bala limestone. Such is the section (Pl. XXXII. fig. 9). 
There is one calcareous band which is repeated in the undulations north of Meifod; and there is one caleareous band 
(not I think four, as represented in the Plate) at Craig-y-glyn. The question proposed by myself was this. Do the 
calcareous bands belong to two, or to one group? My friend decided (erroneously, as is now, I believe, admitted) that 
between Meifod and Craig-y-glyn there were two groups—Caradoc and Llandeilo. But if this were true, and if the typical 
Llandeilo sections were also true, the Meifod and Craig-y-glyn beds must be, geologically, several thousand feet above 
the Bala limestone. This seemed an unnatural conclusion, both physically and palontologically. For the natural 
conclusion was that the Bala, Meifod, and Craig-y-glyn beds must be very nearly on the same horizon. And had the 
Berwyn beds been in the position erroneously represented in the Silurian Section (Plate XXXII. fig. 9), (and without 
the intervention of the great down-cast fault on their north-western side), the conclusion would not merely have been 
probable and natural, but absolutely inevitable. I did not conceal this difficulty; but plainly laid it before my friend 
when we made our traverse over the Berwyn chain to the Bala limestone: and so far from misleading him (in 1834) as to 
his typical Lower Silurian sections, and inducing him to believe in their truth “on my authority,” I shewed him very 
good reasons for suspecting that they might be fundamentally erroneous. But during the five succeeding years he failed 
to follow out the conclusions to which the Berwyn sections seemed very naturally to point; only because he believed in 
the truth of his Llandeilo sections. For if the calcareous and fossiliferous groups on the east side of the Berwyns were 
the equivalents of the Llandeilo rocks, and were also the equivalents of the Bala limestone, it then followed, on evi- 
dence not short of demonstration, that the Llandeilo rocks had been put in a false relation to the Cambrian groups of 
Caermarthenshire and Radnorshire. This is the most remarkable instance, in the history of British Paleozoic Geo- 
logy, of the manner in which the progress of truth may be arrested by an erroneous classification, and a premature 
nomenclature. 
