548 
marcation by which it is to be finally subdivided. For example, the 
lower carboniferous limestone, and the carboniferous slates of Ire- 
land, appear to overlap and descend below the base line of the car- 
boniferous series of England: and the same remark appears to be 
applicable to the lowest beds of the carboniferous series of Seotland. 
And there are similar difficulties in determining the best base line 
for the old red sandstone, as appears from sufsequent details. 
Sections of North Wales, &¢.—The author next discusses two 
sections illustrating the structure of North Wales. One is drawn 
from the Menai Straits, in a direction about E.S.E., so as to cross 
the Berwyn chain and end in the carboniferous series near Oswestry. 
The other is drawn from the Berwyn chain to the carboniferous 
limestone range on the north side of Denbighshire. Fhe greater 
portion of the first section crosses the older beds (the Cambrian 
system) which strike towards the N.E. The other section intersects 
the upper series (Silurian system) which strike towards the N.W., 
passing (in some places unconformably) round the beds of the older 
system. From a consideration of the whole evidenee the rocks are 
grouped in the ascending order, as follows :— 
1. Chlorite slate, quartz rock, and mica slate of Anglesea and 
Caernarvonshire. These are placed at the base of the section: and 
form a distinct class ; and nothing is discovered in this part of the 
section which is perfectly analogous with the Skiddaw slate, or first 
Cumbrian group, to be after deseribed. 
2. The old slate series of Caernarvonshire and Merionethshire, 
alternating indefinitely with bands of porphyry and felspar rock : 
the group is of enormous but unknown thickness, and is bent into 
great undulations, the anticlinal and synclimal lines of which are 
parallel to the strike of the chain. Through wide tracts of country 
it is without fossils ; but at Moel Hebog, Snowdon, and Ghder Fawr, ' 
encrinites, corals, and a few species of bivalves have been discovered 
in it. It ends with the calcareous beds which range from Bala ta 
the neighbourhood of Dinas Mowddy. This is ealled the Lower 
Cambrian group. 
3. The next group (the Upper Cambrian group) commences niet 
the fossiliferous beds of Bala, includes all the higher portion of the 
Berwyns, and all the slate rocks of South Wales which are below 
the Silurian system. Its slate beds are less crystalline, and its 
general structure is more mechanical, than the preceding group, and 
it contains incomparably more fossils, which (though there are many 
extensive portions of the group without fossils) are disseminated 
through the more calcareous beds m great abundance. Many of 
the fossils are identical in species with those of the lowest divisions of 
the Silurian system, nor have any true positive epelaereal characters 
of the group been well ascertained. 
In many parts of South Wales it is separated oa the Silurian 
system by great faults and derangements of the strata, marked by a 
broad band of rotten non-fossiliferous schist. At the north end 
of the Berwyn chain it appears to pass by insensible gradations 
