INTRODUCTION. xlv 
slates associated with the Bala limestone*, that the former groups might remain Silurian 
while the latter group was to be packed in the Upper Cambrian Series of South Wales? 
A single traverse, from Glyn Ceiriog to the northern end of the Berwyn chain, immediately 
south of Corwen, would have settled this question on evidence not short of a physical 
demonstration, and proved that any such separation was impossible. But we did not make 
this traverse: nor did I know, from my previous labours of 1832, that, in point of evidence, 
it was of such vital importance. I did not however blink the difficulty; and I 
guided my friend (as he in his Silurian country had guided me) over the Berwyn chain to 
the Bala limestone, along the high road from Rhaiadr to Bala. We made no mistake in 
the interpretation of the section. We first saw the limestone (near the water-shed) dipping 
west—then forming the base of a great trough containing strata several thousand feet in 
thickness—and lastly emerging at the western outskirt of the chain with its true eastern 
dip. We examined this limestone and collected its fossils; and my friend then declared 
that the Bala limestone was no part of his Silurian System. Was this conclusion derived 
from any peculiarity in the fossil evidence of the Bala rocks? Certainly not: for the 
Bala fossils belonged to a paleontological group that was apparently identical with that of 
Meifod or Glyn Ceiriog. Of this there never was started any doubt. The conclusion was 
derived from the sections which showed several thousand feet of strata above the Bala 
limestone; while no such strata were developed over the Caradoc or Llandeilo groups of 
the Silurian country we had been previously examining. Perhaps, also, I may have influ- 
enced my friend’s judgment on the point, by explaining to him that the Bala limestone 
(though mineralogically lost in the range of the groups through South Wales) was not 
far from the base of the great undulating groups which extended, on the north-west 
side of his Silurian hills, as far as the shores of Cardigan Bay, and collectively formed 
the great group I have called Upper Cambrian. Be this as it may, the explanation I gave 
was correct, and the conclusion my friend arrived at was not based, or professed to be 
based, on palzontological evidence. It was absolutely and simply based on the evidence of 
the sections. 
* Under the name Bala limestone I would now include, as a subgroup, both the Bala and Hirnant limestone; and 
all, or nearly all, the calcareous slates of Glyn Ceiriog, commencing with the lower of the two calcareous bands. Under 
the same name might be included all, or nearly all, the contorted beds near Meifod. One of the present desiderata in 
geology is a good subdivision of the great Bala group of the Tabular View. In North Wales it admits of fowr good sub- 
divisions. The first and lowest, distinguished by its dark pyritous slates, ought to have a distinct name; and from one 
of its localities it might be called Sub-Arenig. It contains in some places many graptolites, and is perhaps the equivalent 
of the “ Moffat-group” of Scotland. 
The second is not so well defined in mineral structure. Among many other Cambrian species it contains the (solitary) 
Ophiura Salteri, and an abundant species of Asterias. (Moel y Garnedd.) 
The third is the Bala limestone subgroup. 
The fourth includes all the higher beds, till they are overlaid, or cut off unconformably, by the May Hill sandstone of 
the Berwynus. But in South Wales (though the lower subgroups are perhaps less perfect) the upper subgroups above 
the parallel of the Bala limestone are more perfect, and far more extensively developed than in North Wales, before they 
reach the representatives of the May Hill beds at the true Silurian base. It is perfectly ruinous to any distinct conception 
of the Upper Cambrian Series to merge all these subgroups under one defined name, such as “ Llandeilo flag.” 
