53. Section across the Lower Silurian volcanic series in the south of Ayrshire (B. N. Peach) 197 54. Section of part of the Arenig volcanic group, stream south of Bennane Head, Ayrshire 198 55. Flow-structure in the lowest felsite on the track from Llanberis to the top of Snowdon 211 56. Section of Snowdon 212 57. Section across the Berwyn Hills. (Reduced from Horizontal Section, Geol. Surv. Sheet 35) 219 58. Section of the strata on the shore at Porth Wen, west of Amlwch 223 59. Section of intercalated black shale in the volcanic series at Porth yr hwch, south of Carmel Point, Anglesey 224 60. Green slates overlain with volcanic breccia, Carmel Point 224 61. Blue shale or slate passing into volcanic breccia east of Porth Padrig, near Carmel Point 225 62. Section of felsites in the Coniston Limestone group, west of Stockdale 232 63. Fine tuff with coarser bands near Quayfoot Quarries, Borrowdale 234 64. Diagram of the general relations of the different groups of rock in the Lower Silurian volcanic district along the western shore of Lough Mask 253 65. Veins and nests of sandstone due to the washing of sand into fissures and cavities of an Old Red Sandstone lava. Turnberry Point, Ayrshire 283 66. Ground-plan of reticulated cracks in the upper surface of an Old Red Sandstone lava filled in with sandstone. Red Head, Forfarshire 284 67. Section across the volcanic series of Forfarshire 286 68. Section across two necks above Tillicoultry, Ochil Hills 288 69. Section of the granite core between Merrick and Corscrine 290 70. Section across the three Dirrington Laws, Berwickshire 291 71. Section of Papa Stour, Shetlands, showing sill of spherulitic felsite traversing Old Red Sandstone and bedded porphyrites (Messrs. Peach and Horne) 292 72. Section across Northmavine, from Okrea Head to Skea Ness, Shetland, showing dykes and connected sill of granite and felsite (Messrs. Peach and Horne) 292 73. Section at the edge of one of the bays of Lower Old Red Sandstone along the northern margin of Lake Caledonia, near Ochtertyre 295