XXxiV PREFACE. 
Sections; by his admirable and ready knowledge of the characteristic fossil species we 
obtained from them. The hypothesis (supra, page xx), either of an overlap somewhere along 
the unconformable junctions of the Cambrian and Silurian rocks of Denbighshire, or of a 
mistake committed by myself in naming some of the fossiliferous groups which appear near 
that junction, was proved at once to be without meaning. I had made no mistakes of the 
kind, nor did we find any great mistake in any of my old sections among the mountains 
of Caernarvonshire and Merionethshire. My work of 1831—32 was right in principle, and 
withstood our renewed test. We examined in great detail the two lines of the Bala 
Limestone, caused by synclinal flexure, securing our work by tracing both beds along their 
strike, and in this way we demonstrated, that the more eastern limestone bands in the 
Llanwddyn valley were identical with the eastern bands that cross the road, between Bala 
and Llangynog, as before stated. We also carefully mapped a part of the country east 
and north of the Northern Berwyns; and we completed in great detail sections which 
connected the Silurian rocks south of the Tannat, and north of the Ceiriog, shewing the 
emergence of the old Cambrian rocks which pass through the intervening country and 
form the highest crests of the Berwyns. We also examined the great fault SE. of 
Llanwddyn, which produces an entire inversion of the strata through a range of several 
miles. This fact I had first observed in 1832, and had verified it by following the inverted 
beds along their strike till they had regained their normal position, and we found that 
we had no corrections to make in this portion of my old Sections of 1832. I mention 
these facts only to shew how conscientiously our work was done. We sought the truth, 
and would have embraced it, to whatever conclusions it might lead us. 
The work done by Mr Salter and myself in 1842-43 seemed to bring to a happy end 
my labours among the higher mountains of North Wales; for I had re-examined all the 
essential parts of my old Sections, and all my groups of strata, assisted by Mr Salter in 
the field, and still more in the closet, by his lists of the fossils we had collected. There 
was no great or fatal mistake in any of my older details, and we came away rejoicing 
in the thought that we had done our work effectually and to a good purpose. But a 
very hard task remained: how were we to join our detailed work to that of the Silurian 
System? It appeared evident, at a glance, that the two were on some points incompatible. If 
our work were true, there must be some very great error lurking among the Lower Silurian 
Groups. How was it to be discovered? I meant to have undertaken the task myself the next 
year: but a serious illness compelled me to spend the summer of 1844 at one of the 
German baths, and in no part of that summer, or of the autumnal months, was I capable 
of taking the field as a Geologist. But my young friend and fellow-labourer, Mr Salter, 
had, with Mr Sowerby’s consent, a commission from myself to examine the Llandeilo Flags 
