302 
rOPULAR SCIENCE REVIEW. 
well known, but tlie fourth (Llanvirn group) is now introduced 
for the first time, though its position has been partially defined 
in previous papers.* The groups are named, in ascending 
order, Arenig, Llanvirn, Llandeilo, and Bala. 
Arenig group. — This name was first given by Prof. Sedg- 
wick, to a group of rocks well developed in the Arenig district 
in North Wales. For many years, however, it yielded but few 
fossils, and its importance was scarcely suspected until after 
the discovery in it, at St. David’s, of a very rich fauna. In 
a former paper I divided the group into lower, middle, and 
upper parts. I have since found that the upper, as there 
defined, is so distinct in its fauna from the underlying or 
middle part, and, indeed, at present so little known out of the 
neighbourhood of St. David’s, that I have, at the suggestion of 
Prof. Lap worth, given it a local name, and will therefore 
describe it as a distinct group. The Arenig proper will, 
therefore, now be divided into two parts, upper and lower, 
the former being the portion previously described as the middle 
part. The group consists almost entirely of black slates, with, 
however, a few beds of flaggy sandstones, and in North Wales 
with some grits. Graptolites, which appear for the first time in 
the underlying Tremadoc rocks, f occur in this group in great 
abundance ; the trilobites, also, are altogether unlike those in 
the underlying rocks. The evidence, therefore, of a palaeonto- 
logical break between the Cambrian and Ordovian, is tolerably 
clear, though stratigraphically it is not seen. The so-called 
‘ Skiddaw slates ’ belong chiefly to the upper portion of this 
group. The graptolites hitherto collected in the lower portion 
are chiefly Diplograpti and Cladophora; in the upper, the genera 
Tetragraptus and Didymograptus . 
sJ 
g g J 
" Upper. | 
is] 
Lower, j 
Slates, Flags, and Sandstones, with JEglina, Calymene , 
&c. 
Fine Black Slates and Shales, with Ogygia, Trinucleus , 
& c. 
The view, Pl. VIII., fig. 4, shows the Arenig rocks at White- 
sand Bay. A few beds of the Tremadoc group are also seen, but 
most of the beds of this group have been here cut ofi by a 
fault. 
Llanvirn group. — As stated above, this group was described 
by me formerly as a portion of the Arenig (its position is 
seen in Sect. 2, p. 308). It consists of' a thick series of 
* Hicks, Quar. Jour. Geol. Soc. May 1875. 
t Found by Dr. Callaway in Shropshire, and also by Mr. Homfray near 
Portmadoc. 
