xlii INTRODUCTION. 
a group of sandstone and conglomerate which forms the base of the slates and flagstones 
of Denbighshire (afterwards called “Upper Silurian.”) 
During the same summer I also completed a series of detailed sections among the 
dislocated and contorted groups on the east side of the Berwyns; where (in consequence of 
enormous, but local, dislocations) the north-eastern strike of the true Cambrian groups 
entirely disappears. But I made no attempt to map, in any detail, that part of the country. 
The task was reserved for the work of another year. 
It was during the same summer that I first saw the great importance of the Bala 
group, and its discordant relations to the overlying series (Denbigh flag, &c.) which is now 
called Silurian. The Bala limestone seemed, both in its sectional place and in its fossils, 
to represent the Coniston limestone of the lake mountains of the north of England: and 
these two calcareous bands seemed to derive a new importance from the fact, that each of 
them very nearly defined the epoch when there was an entire cessation of those contempo- 
raneous plutonic eruptions which had such a marked effect upon the older Cambrian 
deposits both of Wales and Cumberland. 
Lastly, at the end of a summer of very great labour, I made two or three long and 
rapid traverses through the older rocks of South Wales; in the hopes of finding by the 
prolongation of the prevailing strike, and especially by the prolongation of the Bala lime- 
stone, a base line for the operations of another summer. In these hopes I was disap- 
pointed; for the Bala limestone loses its distinctive type, and the strike among the older 
rocks of South Wales is broken and disjointed: the region having been crushed between 
two great forces of elevation—one represented by the north-eastern strike—the other (of a 
much later period) represented by the east and west strike which produces such a vast 
impress upon the physical features of South Wales, affecting not only the Cambrian and 
Silurian, but also the whole Carboniferous series. 
After the summer of 1832 my health broke down, and I was incapable of taking up 
my task, at the time I intended, in 1833; but spite of this great disappointment I had 
done enough in 1832 to establish a conclusion from which I have never flinched, and 
which I believe indisputably true: viz. that the South Welsh equivalents of the Bala lime- 
stone and its calcareous slates have their true geological place near the base of those 
great groups which roll, in countless and unsymmetrical undulations, from the shores of 
Cardigan Bay, towards Mynydd Epynt, and other ranges of the comparatively low Silurian 
hills. I affirm, therefore, with the confidence of perfect truth, and as a conclusion to the 
previous remarks, that at the end of the summer of 1832 I had so far overcome the four 
difficulties above-mentioned, as to have put the Cambrian groups in their true co-ordination. 
The detailed sections I exhibited to the Geological Society in 1838, in illustration of 
a Paper, of which an abstract is given in their Proceedings (Vol. 11 p. 679, &c.), were all 
copied from my field sketches of 1831 and 1832: and so far as regards the country to 
the west of the Berwyns, nearly all the sections published by Mr Warburton*, in illustration 
* Quarterly Journal of the Geological Society, Vol. 1. pp. 5—22. 
