CLASSIFICATION OF THE SILURIAN ROCKS. 25 
lowest beds of the series termed Silurian by American, German, French, Russian, 
Swedish, Norwegian, and Spanish authors. 
This band, the “ Primordial Zone” of Barrande, is manifestly the natural base of the 
Silurian or earliest great Trilobitic and Brachiopodous series. But, whilst the strata 
passing beneath the Stiper Stones of Shropshire are merely consolidated mudstones, 
they are in North Wales hard slaty rocks. 
The next overlying strata are known in Wales as the 'Tremadoc Slates. The relations 
as well as the fossils of these deposits, whether in North or South Wales (Pembroke- 
shire), have been ably worked out of late years by Mr. Salter; whilst their general range 
and characters are delineated by Professor Ramsay, in his classical and valuable Monograph 
of the North-Welsh Rocks, just published,* and in which the general classification of 
which I am now tracing the outline is adopted throughout. 
This band, the Lower Llandeilo of my classification (see ‘ Siluria, 2nd ed., passim), 
is everywhere rich in fossils, being characterised by Lingule, Dictyoneme, Crustacea 
(Trilobites), and Heteropoda. ‘The Trilobites specially characteristic of this zone 
are, Psilocephalus innotatus, Mobe Homfrayi, Asaphus Homfrayi, Ogygia scutatria, 
and Conocoryphe, associated with Dictyonema (Graptopora) sociale, Conularia, and 
Theca. 
The associated sandstone strata have been altered in the Stiper Stones of Shropshire 
into quartzites and hard schists, and exhibit on their western flank a peculiar group of 
fossils, viz., Zrinucleus Murchisoni, Aylina binodosa, Lingula plumbea, Redonia (Cucullella) 
complanata, Didymograpsus geminus, Orthis alata, and Orthoceras Avelinit. 
Ascending in the series, the chief or upper part of the Llandeilo Formation is essen- 
tially argillaceous—mere mudstones in one district, calcareous flagstones in another, and 
slates in a third; and is characterised by numerous Brachiopoda, particularly by Orthide, 
Rhynchonellide, and Lingulide, but specially also by Trilobites (Ogygia Buchii, Asaphus 
tyrannus, Calymene Murchisoni, Dionide atra, Aiglina caliginosa, Trinucleus Murchisoni), 
and several genera of Graptolites, which first make their appearance in the Lower 
Llandeilo. The last-named Zoophytes highly typify, indeed, the whole Silurian system, 
except the “Zone Primordiale,” and are never found in younger rocks. A few Echino- 
dermata also occur in the Lower Llandeilo Beds; and here we have also the first well- 
determined evidence of the occurrence of the Lamellibranchiata, characterised by Palearca, 
Ribeiria, Redonia, &c. The Llandeilo Formation, with occasional limestones, occurs not 
only in Siluria and Wales, but also in Cumberland, where, as above stated, the Skiddaw 
Slates, its representative, form the lowest band in that slaty region. 
The next ascending formation, the Caradoc, is in Shropshire, to a great extent, a 
sandstone, associated with some impure shelly limestones and shale; but in North Wales, 
after certain folds, it occupies the Bala country, and becomes a hard and slaty rock, some- 
1 «Memoirs of the Geological Survey of Great Britain,’ vol.iii; “On the Geology of North Wales.”’ 
4 
