xvi PREFACE. 
following nearly East and West sectional lines, the deposits were discovered in their true 
relations and sectional order: and commencing nearly on the line of the Holyhead road, I 
marked all the anticlinal and synclinal fiexures upon a section copied from nature. The vast 
undulations of the strata, the constant intrusion and alternation of igneous rocks, at first 
threw some difficulties in my way; but I had learned to encounter them during my previous 
three years’ labour among the Lake mountains, and I did not find any insurmountable diffi- 
culties in reading the succession given by each section. In this way I made three parallel 
nearly East and West traverses. The first from the shores of the Menai, by the great 
Penrhyn slate-quarries, and so on, by the summits of the high mountains, over the top of 
Glyder Fawr to the neighbourhood of Capel Curig: the second, by the mountains South of 
Llanberis, over the top of Snowdon, and thence into the valley above Beddgelert: the third, 
by a parallel mountain-track, passing over Moel Hebog and ending in the valley below 
Beddgelert, a little farther to the South-west. 
On a careful revision of the several sections presented on these three lines, I found that 
even in the most complicated curves the chief anticlinal and synclinal lines might be brought 
into an approximately close comparison, so as greatly to assist in the construction of a 
general section of the country and the establishment of good physical groups. Thus the 
great synclinal trough of Cwm Idwal, on the first line of section; another synclinal in the 
second line, within which rests the top of Snowdon; and a third synclinal which underlies 
the top of Moel Hebog: all affect one group of strata, through which pass the most 
remarkable fossiliferous beds of the whole Cambrian Series. 
Taking this as a kind of key-note to guide me in further discoveries I examined the 
sections in more detail, and made out the relations of their corresponding parts; and 
before the working season was over (in 1831) I completed an approximate Geological Map 
from actual survey of nearly the whole of Caernarvonshire. No names were of course given 
to the natural groups of strata; but the part some of them played in the physical de- 
velopment of the country was, I thought, clearly established. 
I have given these details to shew the honest and very laborious way in which I set 
about my work. I had, in truth, little difficulty in reading the sections, as their language 
was written in characters almost identical with those I had long studied in the Cumbrian 
mountains. 
The base-line on the shores of the Menai was broken and imperfect; and its associa- 
tion, made in after years by the Government Survey and by myself, with the Longmynd 
rocks, was, I think, partly hypothetical. But along each line of section, above described, I had 
found a magnificent, and, on the whole, an ascending series of deposits, of grand features and 
of enormous thickness, 
