Since the days when Sedgwick and Murchison first worked out the succession of Palaeozoic strata in North Wales, there has always existed a considerable difference of opinion as to the best method of subdividing the Cambrian-Silurian strata. Later research has shown that the rocks included by Sedgwick in his Cambrian system, fall naturally into two groups; for the upper of these Prof. Lapworth has suggested the term Ordovician, from the name of the Ordovices, who inhabited a part of northern Wales. At the base of the system we have a series of volcanic and sedimentary rocks to which Sedgwick gave the name Arenig; above these there occur the Llandeilo Flags, succeeded by a considerable thickness of rocks known as the Bala series. The rocks making up these Ordovician sediments consist for the most part of slates, sandstones and limestones with volcanic ashes and lavas. Much of the typical Welsh scenery owes its character to the folded and weathered rocks laid down on the floor of the Ordovician sea, on which from many centres of volcanic activity lava streams and showers of ash were spread out between sheets of marine sediment. The Arenig Hills, Snowdonia, and many other parts of North and South Wales, parts of Shropshire, Scotland, Sweden, Russia, Bohemia, North America and other regions consist of great thicknesses of Ordovician strata. IV. =Silurian.= Passing up a stage higher in the geologic series, we have a succession of conglomerates, sandstones, shales, and limestones; in other words, a series of beds which represent pebbly shore deposits, the sands and muds of deeper water, and the accumulated débris of calcareous skeletons of animals which lived in the clear water of the Silurian sea. The term Silurian (Siluria was the country of Caractacus and the old Britons known as Silures[51]) was first applied by Murchison in 1835 to a more comprehensive series of rocks than are now included in the Silurian system. The rocks of this period occur in Wales, Shropshire, parts of Scotland, Ireland, Scandinavia, Russia, the United States and other countries. After the accumulation of the thick Ordovician sediments, the sea-floor was upraised and in places converted into ridges or islands of land, of which the detritus formed part of the material of Silurian deposits. The limestones of the Wenlock ridge have yielded an abundant fauna, consisting of corals, crinoids, molluscs and other invertebrates. In this period we have the first representatives of the Vertebrata, discovered in the rocks of Ludlow. In fact, in the Silurian period, “all the great divisions of the Animal Kingdom were already represented[52].” V. =Devonian.=