PREFACE. XV 
sification and nomenclature given by Mr Salter on page 25. I have a similar, more simple 
comparative Table, kindly given me by Prof. Morris, which I will not here copy in extenso, as 
I wish this prefatory notice to be short; but I will copy the first column of his Table, which 
he believes to represent my scheme of arrangement as well as his own; and with which 
I wish to make no change; except that I would add the Llanberis Group to the Longmynd 
and Bangor Groups, and remove the Harlech Group to the base of the Menevian, as before stated. 

| Ledbury Shales. 
Upper Upper Ludlow Group. 
Siunian | Lower Ludlow 5 
[ Wenlock - 
Lower | Lower Wenlock __,, 
May Hill 55 
Upper Bala re 
, Upper Middle Bala - 
tL Lower Bala . 
Arenig or Skiddaw ,, 
C b Pic g d 
ambrian | aq Ale Tremadoe “5 
Ffestiniog 9 
Menevian Be 
Wace Harlech or Bangor ,, 
Longmynd 5 
If it be asked how the great succession of our Older Paleozoic Rocks was determined, 
I can only refer to my own individual labours, carried on during many successive years 
among the older rocks of England. 
I commenced my task in Wales in 1831, accompanied for a short time by my friend 
Charles Darwin—a name now well known and honoured in the whole world of science—but 
other engagements soon drew him away from N. Wales. We laid down the northern boundary 
of the Carboniferous Limestone together, and I at first purposed to examine the rocks in 
descending order. But I found in Denbighshire the same interrupted broken masses of Old 
Red Sandstone which [had so many times noticed between the Carboniferous and the Silurian 
and Cambrian rocks in the North of England. I therefore despaired of establishing a good base 
to work upon in that district; and after a short examination of the rocks on both sides of 
the Menai Straits, I resolved to fix a provisional base-line on the Caernarvon side of the Straits. 
I soon found that the prevailing strike of the country was about N.N.E.; therefore by 
