XX INTRODUCTION 
I. Lower Paleozoic Division, representing the Cambrian and Silurian Series in 
ascending groups. 
a. Longmynd slates, &e. 
1, Longmynd and Bangor group f b. Llanberris slates ; ; alternations of roofing-slates and grits. 
(Pave Gemoun) ant Harlech grits; sometimes approaching a conglomerate form. 
a. Lingula flags. 
Se 6. Tremadoe slates. 
2. ¥ po rel is c. Arenig slates and porphyries; Festiniog slates; slates, flags, and 
igi eambean) grits; indefinite alternations of porphyry and trap-shale: one irre- 
gular band of limestone near the top of the group. Thickness great. 
. Lower Bala rocks. (1) A great zone of dark and sometimes 
earthy slate. (2) A great series of slates, flags, and grits, 
ascending to the Bala limestone. 
6. Upper Bala rocks. Under this term are included —(1) the Bala and 
| Hirnant limestone and the Llandeilo calcareous flag—(2) Flag- 
| stones, slates, calcareous beds, and shelly sandstones of Caer 
| Caradoc. The whole series ending, in North Wales, with slate 
l 
Cambrian Series. 
3. Bala group 
(Upper Cambrian) 
and flagstone; and, in South Wales, with slates, grits, and 
coarse conglomerates. The group of very great thickness. 
Immediately above these three groups there is a great change of physical conditions. 
The most characteristic and abundant of the older organic types disappear, and new types 
take their place. The sections are usually broken and discontinuous; and the upper (or 
Silurian) groups sometimes overlap the lower (or Cambrian) groups unconformably. Here, 
therefore, (to adopt a language in common use), we have the commencement of a new System. 
Without counting the vast thickness of the Longmynd slates, the thickness of the Cambrian 
Series, where well developed, is, I think, more than 25,000 feet. 
Lower Paleozoic Division continued. 
. May Hill sandstone ; Pentamerus (or Norbury) limestone. 
a 
1. Wenlock group. 6. Woolhope (or Lower Wenlock) limestone. 
c 
g . Wenlock shale. 
D ne Upper (or great) Wenlock limestone. 
a (a. Lower Ludlow rock. 
3 2. Ludlow group..... | 6. Aymestry limestone. 
2) ¢. Upper Ludlow rock. 
d, Tilestone. 
The aggregate thickness of the series about 5000 * feet ? 
* IT obtain this number from an estimate of the thickness of the “Upper Silurian” rocks, by Dr Fitton, added to 
the thickness of the May Hill sandstone, as given by Professor Phillips. Aggregates of this kind are frequently too 
great. For, as a general rule, where one of a set of connected groups rises above the average thickness, another group 
will probably descend below it. For example. The Woolhope limestone of Presteign is a noble rock, but the Wenlock 
limestone is there quite degenerate. At Wenlock the limestone forms a grand terrace, but the Aymestry limestone has 
almost vanished. At Leintwardine the Aymestry limestone is a grand rock, but the Wenlock limestone is degenerate. 
Many other examples might be quoted. 
