INTRODUCTION. xli 
In conformity with the same general views, we might describe (and certainly with 
approximate truth) the conglomerate and sandstone beds (now called May Hill Sandstone) 
which form the true Silurian base, as an ancient shore, or shallow-sea deposit, mantling 
round the deposits of a more ancient elevation, and marking the commencement of a perfectly 
new epoch. And, spite of the great movements and dislocations which have modified and 
broken up the continuity of this ancient shore, the peculiar place it once held among our 
paleozoic rocks is not yet obliterated from the Geological map. I wish not, however, to rest 
on speculation, but on facts; and to them I return. 
My work in 1831 was almost confined to Caernarvonshire, the whole of which I 
endeavoured to map, laying down all the larger bosses of protruded porphyry; and, as far 
as possible, all the anticlinal and synclinal lines. This map, with its system of parallel undu- 
lations, along with a transverse section, was exhibited and described in a lecture before the 
British Association in June, 1832. On that occasion I dwelt especially on the position of the 
fossiliferous trough of Snowdon, and on its strike along the Caernarvon chain; and [ 
shewed, on the unequivocal evidence of sections, that the fossil-beds were many thousand 
feet above the great slate-quarries of Nant Francon and Llanberris*. 
Before the expiration of 1832 I had traced two parallel sections from the Menai Straits 
to the east side of the Berwyn chain as far as the groups of the Carboniferous series; and 
I had completed one section which appeared to include every group, above the great 
Merioneth anticlinal, up to the limit of the Cambrian series near Corwen; not far from which 
place (but amidst enormous and complicated dislocations) this series seemed to pass under 
also that it is often suggestive of the right points towards which new observations ought to be directed; and, on that 
account, an efficient help towards new generalizations and discoveries. What has been the cause of this parallelism (in 
the cases where it does occur) is a question of physics rather than of geology. 
* I never considered (as is erroneously stated by the author of “Siluria,” p. 9,) the fossiliferous trough of Snowdon 
“to lie near the bottom of the so-called Cambrian rocks!” I proved the contrary by the direct evidence of sections: 
viz. that commencing with the (Harlech) grits over the great Nant Francon or Llanberris quarries there was an ascending 
series through more than two miles of highly inclined beds before we came to the fossiliferous trough of Cwm Idwal or of 
Snowdon. This was one of the main points of my lecture in June 1832, and my friend Murchison was, if I mistake not, 
present when I gave it. I had no means, at that time, even of guessing what were the beds which overlaid the Snowdonian 
fossil-beds in Merionethshire, and I gave no conjectural opinion on the subject. I did however then identify the beds at 
the top of Snowdon with the fossiliferous beds farther south near Bodean. 
When I afterwards (during the same summer) made out the relations of the Bala group, I had great doubts about the 
relative position of the fossiliferous slates of Snowdon. The Bodean fossiliferous beds seemed to be identical with those of 
Bala. Were then the Snowdonian fossil-beds on the parallel of the Bala limestone? There were three objections against 
a positive reply to this question. (1) Counting from the Harlech grits to the Snowdonian trough, there was a far less 
thickness of beds than there was from the same grits near the Merioneth anticlinal up to the Bala limestone. (2) Over the 
Snowdon fossils there appeared (as I constructed the sections) a far greater thickness of beds than I,could make out from 
any section over the Bala limestone. (3) The Snowdonian fossils were both embedded amongst, and overlaid by, con- 
temporaneous plutonic rocks; but the Bala limestone seemed to be above all the plutonic rocks; in which respect it pre- 
sented a good analogy to the Coniston limestone. These three facts of observation did not, however, amount to a proof 
either positive or negative. Hence I continued in doubt about the exact age of the fossil-beds of Snowdon. And (so far 
as I remember) I never afterwards made use of them in any single section in which I endeavoured to eliminate the suc- 
cessive Cambrian groups. In not one paper did I eyer make use of them as a point of departure which assisted in the 
definition of the Cambrian groups. The “erroneous idea” attributed to myself (“Siluria,” p. 9,) is but the erroneous idea 
of the author. 
f 
