54. Erratic Blocks 147 55. Ancient Glaciers of England, Ireland, Scotland, and Wales 150 56. The Glacier Epoch 152 57. Glacial Theories 155 58. Dilatation and Sliding Theories 155 59. Plastic Theory 156 60. Viscous Theory 161 61. Regelation Theory 163 62. Cause of Regelation 167 63. Faraday's View of Regelation 171 64. The Blue Veins of Glaciers 176 65. Relation of Structure to Pressure 183 66. Slate Cleavage and Glacier Lamination 187 67. Conclusion 191 [Illustration: CLOUD-BANNER OF THE AIGUILLE DU DRU (par. 84 and 227).] THE FORMS OF WATER IN CLOUDS AND RIVERS, ICE AND GLACIERS. § 1. _Clouds, Rains, and Rivers._ 1. Every occurrence in Nature is preceded by other occurrences which are its causes, and succeeded by others which are its effects. The human mind is not satisfied with observing and studying any natural occurrence alone, but takes pleasure in connecting every natural fact with what has gone before it, and with what is to come after it. 2. Thus, when we enter upon the study of rivers and glaciers, our interest will be greatly augmented by taking into account not only their actual appearances, but also their causes and effects. 3. Let us trace a river to its source. Beginning where it empties itself into the sea, and following it backwards, we find it from time to time joined by tributaries which swell its waters. The river of course becomes smaller as these tributaries are passed. It shrinks first to a brook, then to a stream; this again divides itself into a number of smaller streamlets, ending in mere threads of water. These constitute the source of the river, and are usually found among hills. 4. Thus the Severn has its source in the Welsh Mountains; the Thames in the Cotswold Hills; the Danube in the hills of the Black Forest; the Rhine and the Rhone in the Alps; the Ganges in the Himalaya Mountains; the Euphrates near Mount Ararat; the Garonne in the Pyrenees; the Elbe in the Giant Mountains of Bohemia; the Missouri in the Rocky Mountains, and the Amazon in the Andes of Peru.