In the earliest or Ordovician period a sea with islands scattered over it occupied the British area. Near the shores sands and muds accumulated to form sandstones and shales. In the clearer waters shell-fish of obsolete types abounded, and from their dead shells were formed bands of limestones. Corals, too, were present in this sea, which goes to prove that our islands were then bathed by warmer waters than those which surround them to-day.

Great volcanic activity prevailed at this time. In Wales and Cumberland, in counties Dublin and Kildare volcanoes existed. In Waterford (Bed and Breakfasts, Waterford, Ireland) we have a grand series of volcanic rocks which burst their way through the sea-floor and mingled their lavas and ashes with the sands and muds, causing destruction among the shell-fish and coral polyps. Slates of Ordovician age are found in the vicinity of Waterford.

The Silurian period was a more peaceful time here and in the British area generally, but in the Dingle peninsula the final struggles of these plutonic forces were enacted. At Clogher Head, and along the coast north and south, the Atlantic has laid bare old lavas and ashes that were poured out upon the bottom of the sea. Now and again came paroxysmic eruptions sufficient to blot out of existence the abundant animal life of those waters, but it soon returned, and so we find sediments again forming on top of an old lava flow. In addition to this district already mentioned, Silurian rocks are also found on the plateau between the Commcragh mountains and Portlaw, in the district cast of Slieve-namann and in the region west of the Galtec mountains. The highlands west and south of the Devils Bit reveal large areas of Silurian rocks, and we find them abundantly displayed in the Arra and Silvermine mountains in Tipperary and also in the Slieve Bernagh and Slieve Aughty mountains west of the Shannon. In all cases their exposure is due to the removal by denudation of the Old Red Sandstone rocks which rested upon them

At the beginning of the glacial period the sea stood at approximately the same level as now. The annual snowfall was greater than the heat of summer could melt, and hence an ice-sheet was formed. This ice-sheet appeared much earlier in the north than in the south. Gradually, however, invasions of ice from the great Central Plain and of Irish Sea ice took place, the former finding a way at first through the passes of the hills, but afterwards mounting and overtopping most of them, the latter occupying and riding over the country along the coasts of Waterford (Accommodation, Waterford, Ireland) and Cork.

Erratics are abundant, especially in the south-west of Munstcr. These are. blocks, often of great size, that have been carried from their place of origin by ice. When they occupy precarious positions they are termed perched blocks. At Cloughlowrish Bridge, in Co. Waterford (Hotels, Waterford, Ireland), is an erratic of Old Red Sandstone resting upon an igneous rock. Near Kenmare is another of sandstone resting upon a limestone surface. The limestone has been artificially cut away all round at a remote period, so that the large erratic now rests on a slender pedestal. On Knockbrack mountain are a large number of them, and about Glengariff and westwards towards Adrigole they are perched in all kinds of positions.

At the end of the Mer de Glace, near Chamounix, are rocks which present a smooth and polished surface and on which striations are also observable. The smoothing has been effected by the sand which the ice contained in its base, and the striations are due to the chiselling action of fragments of rocks held as in a vice by the glacier as it moved forward.

One of the surest forms of evidence concerning the lotion of glaciers is to he found in such smoothed surfaces of rock upon which are striations more or less deeply incised. Some of these begin as fine lines and increase in thickness until they come to an abrupt termination. In this case the striation becomes coarser in the direction in which the. glacier was moving