Whether the horizontality of the shelves or terrace-lines is really as perfect as has been generally assumed is a point which will require to be tested by a more accurate trigonometrical survey than has yet been made. The preservation of precisely the same level in the lowest line throughout the glens of Roy, Spean, and Laggan, for a distance of 20 miles east and west, and 10 or 12 miles north and south, would be very wonderful if ascertained with mathematical precision. Mr. Jamieson, after making in 1862 several measurements with a spirit-level, has been led to suspect a rise in the lowest shelf of one foot in a mile in a direction from west to east, or from the mouth of Glen Roy to a point 6 miles east of it in Glen Spean. To confirm such observations, and to determine whether a similar rate of rise continues eastward, as far as the pass of Muckul, would be most important. On the whole, I conclude that the Glen Roy terrace-lines and those of some neighbouring valleys, were formed on the borders of glacier-lakes, in times long subsequent to the principal glaciation of Scotland. They may perhaps have been nearly as late, especially the lowest of the shelves, as that portion of the Pleistocene period in which Man co-existed in Europe with the mammoth. CHAPTER 14. -- CHRONOLOGICAL RELATIONS OF THE GLACIAL PERIOD AND THE EARLIEST SIGNS OF MAN'S APPEARANCE IN EUROPE--CONTINUED. Signs of extinct Glaciers in Wales. Great Submergence of Wales during the Glacial Period proved by Marine Shells. Still greater Depression inferred from Stratified Drift. Scarcity of Organic Remains in Glacial Formations. Signs of extinct Glaciers in England. Ice Action in Ireland. Maps illustrating successive Revolutions in Physical Geography during the Pleistocene Period. Southernmost Extent of Erratics in England. Successive Periods of Junction and Separation of England, Ireland, and the Continent. Time required for these Changes. Probable Causes of the Upheaval and Subsidence of the Earth's Crust. Antiquity of Man considered in relation to the Age of the existing Fauna and Flora. EXTINCT GLACIERS IN WALES. The considerable amount of vertical movement in opposite directions, which was suggested in the last chapter, as affording the most probable explanation of the position of some of the stratified and fossiliferous drifts of Scotland, formed since the commencement of the glacial period, will appear less startling if it can be shown that independent observations lead us to infer that a geographical revolution of still greater magnitude accompanied the successive phases of glaciation through which the Welsh mountains have passed.