24 INTRODUCTION. 
may be said, that, looking to their diffusion in the counties of Dublin, Wicklow, Wexford, 
Kerry, Cork, Limerick, Galway, and Donegal, they must occupy a large portion of the 
subsoil of that green and drift-covered island, or nearly 7000 square miles. ‘Thus, as regards 
the British Isles, the Silurian Rocks rise to the surface over nearly 33,000 square miles. 
Viewed as they occur, not only in Britain, but in distant parts of the globe, the 
Silurian Rocks must be chiefly regarded as deposits of calcareous. and argillaceous mud 
and sand, accumulated in seas of different and varying depths. So much does the purely 
argillaceous character prevail in the typical districts of the original Silurian Region, that at 
one time I even thought of applying to the group the term of ‘‘Mudstone Series.” But when 
[ ascertained that the same deposits which are purely argillaceous in large tracts alternated 
in others with limestones and sandstones, and that in Wales they passed into crystalline 
slates, the use of a geographical term, which should embrace all the stony varieties of the 
series, became a necessity. Hence the word “Silurian,” as simply implying the name of a 
region in which the true order was first made out. In numerous other regions, as well as in 
England and Wales, the same alternation of lithological conditions was found to exist. 
‘Thus the slightly indurated mud on which St. Petersburg is built was proved to be the 
equivalent of the Lower Silurian rocks of Britain ;' whilst the same deposits in the Ural 
Mountains were found to be highly crystalline schists and slates, the sedimentary sand- 
stones being changed into quartz-rocks. 
Looking to these changes of mineral character in the deposits of the same age, it has 
been found difficult, if not impossible, to assign to each formation or sub-formation of the 
system a lithological description which would invariably be found to apply. Hence the 
utter inapplicability of the old mineralogical term ‘“‘ Grauwacké,” which had indeed been 
indiscriminately apphed by the earlier German geologists to all the rocks which have since 
been separated into Cambrian, Lower and Upper Silurian, Devonian, and even Lower Car- 
boniferous Rocks. ‘The following, therefore, is to be taken as a generalised sketch of the 
stony character of the Silurian Rocks of the British Isles only, and in their ascending order. 
The lowest of the Silurian Rocks of Shropshire, the tract wherein their order was first 
pointed out by myself, is a great mass of dark-coloured schistose shale, or consolidated 
mud, reposng upon the Cambrian Rocks of the Longmynd Mountain, and dipping 
under the picturesque ridge of the Stiper Stones; thus constituting the true base of my 
original system. ‘These shales, or soft schists of finely levigated mud, were next found to 
be the equivalents in position of the Welsh Lingula-flags; and though in Shropshire 
they have not as yet been found to contain fossils, yet in Merionethshire in North Wales, 
and mm Pembrokeshire in South Wales, they not merely contain abundance of Lingula— 
which thenceforward occur throughout the Silurian System, but also Trilobitic 
Crustacea (Paradoxides, Olenus, Agnostus, and Microdiscus), and Hymenocaris; and 
most of these, in North America, Germany, Russia, and Sweden, characterise the 
1! See ‘Russia and the Ural Mountains,’ by Murchison, de Verneuil, and Keyserling, vol. i, p. 24. 
