CLASSIFICATION OF PALAEOZOIC ROCKS OF THE BRITISH ISLES. 301 
Dolgelly group (Upper Lingula flags). — This group consists 
mainly of blue and black slates. It was evidently deposited in 
a tolerably quiet sea, and contains a rich fauna. The beds are 
well exposed in the Mawddach Valley, and about Portmadoc in 
North Wales. The so-called Malvern black shales also belong 
to this group. At St. David’s, fine grey shales chiefly occur at 
this horizon. The Olenoid forms, Parabolina, Peltura, and 
Sphcerophthalmus, characterize this group. 
>h ( Upper, j Soft Black Slates, with Peltura, Sphcerophthalmus, 
g pi I 2 3 °0 ft. | & c . 
s g J 
3| j 
P ^ 3 QQ^' | Hard Blue Slates, with Parabolina, Sec. 
Tremadoc group. — This group consists mainly of dark 
earthy flags, flaggy sandstones, and iron-stained slates. It 
was carefully examined many years ago in North Wales, by 
Sedgwick, Salter, and Homfray, and shown to contain a very 
important fauna. Up to the year 1866, it had only been 
recognized in North Wales ; but in that year I was able to 
announce the discovery at St. David’s of a large series of fossils, 
more nearly allied to those in this group than to any others 
known. It has since been found in Shropshire by Dr. Callaway. 
About sixty species have now been discovered altogether, and 
nearly all are peculiar to the group. Many genera, and several 
orders, appear here for the first time in the succession. The 
Lamellibranchs, Crinoids, Asteroids, and Cephalopods, are not 
known to occur at a lower horizon, and several of these, as 
yet, have only been found at St. David’s, in these rocks. 
o 1* Upper, I Iron-stained Flags and Slates, with Asaphus Homfrayi, 
p \ 300 ft. ( Angelinia, Se c. 
5 § s 
io / Lower, j Bark Earthy Flags and Flaggy Sandstones, with Kes- 
^ ( 200 ft. | euretus, Niobe, Se c. 
2. Ordovian (Lower Silurian). 
The name ‘ Ordovian,’ has been given to this formation by 
Prof. Lapworth, who has paid great attention to its fossils, 
especially to the Giraptolites, and these somewhat specially 
characterize the groups in the formation. I adopt this name 
now in preference to that of Lower Silurian, as less likely to lead 
to confusion, and as being in many respects an appropriate one. 
It consists of four well-marked groups, each with a distinct and 
important fauna. Three of these have been for a long time 
