[55] An abstract of five lectures given at the Royal Institution in January, February, and March, 1884. _Nature_, vol. xxix. pp. 325, 347, 396, 419, 442. The notes here inserted are now added for the first time. [56] See postea p. 154. [57] To these east-and-west foldings of the terrestrial crust we owe the plication of our Cretaceous and older Tertiary strata which have given us the ranges of the North and South Downs. They were accompanied by powerful horizontal thrusts of portions of the crust. The successive plications of the terrestrial crust in the European area have since been discussed by Prof. Suess in his _Antlitz der Erde_. [58] See note on p. 146. [59] A pre-Cambrian group of mountains has been exposed by denudation in the west of Ross-shire. See p. 72. [60] Since these lectures were given the remarkably complex structure of the North-West Highlands has been made known. It has been ascertained that gigantic horizontal displacements of rock have occurred in that region, and that similar 'thrusts' have more or less affected the rest of the Highlands. [61] A striking instance of one of these fragments is to be seen on the summit of Slieve League in Donegal. See p. 61. [62] See pp. 161, 193. [63] Other periods of prolonged denudation might be mentioned. Thus in pre-Cambrian times the Lewisian Gneiss was stupendously eroded before the deposition of the Torridon Sandstone, which in turn was enormously worn down before the Cambrian sediments were spread unconformably over it. The deposition of the Old Red Sandstone was preceded and accompanied by a vast degradation of the pre-existing rocks. [64] That the Secondary formations once extended far beyond the limits within which they are now confined has been impressively demonstrated by the discovery of large masses of Rhaetic, Liassic, and Cretaceous strata in a great volcanic vent of Tertiary age in the Isle of Arran (see Messrs. Peach, Gunn, and Newton, _Quart. Journ. Geol. Soc._, lvii., 1901). It may consequently be inferred that the present drainage system and topographical features of much, if not most, of the country have been established since the time of the Chalk (see the 'Geology of Eastern Fife' in _Memoirs of the Geological Survey_, 1902, p. 281). The stupendous erosion of the Tertiary lava-plateaux of the west of Scotland and the north of Ireland shows, in the most impressive way, how greatly the topography has been carved out since older Tertiary time. [65] I have shown that some of the wildest glens of the Highlands and the deepest dales of the southern Uplands of Scotland have been hollowed out since early Tertiary time by the various streams that still flow in them. _Scenery of Scotland_, 3rd ed., pp. 162, 339, 361. [66] The Scottish Lakes have in recent years been made the subject of detailed study by Sir John Murray and Mr. F. Pullar. The results of their investigations have appeared in the _Geographical Journal_.