Slates, with their ragged edges and resistance to rain, play their part in wilder mountain-scenery. Frost-action destroys them, producing taluses that slip frequently towards the valleys; but the residual crags assume more serrated forms, in contrast with the smooth covering of the lower slopes. The cleavage, when steeply inclined to the horizontal, promotes the cutting of gullies down the mountain-sides, and the intervening ribs of rock may easily be mistaken for uptilted strata. The entrance to the Pass of Llanberis at Dolbadarn is a fine picture of slate-scenery. Eventually, mountains formed of slate assume hog-backed and rounded forms, but they still, where notched by streamlets, yield sheer cliffs and picturesque ravines. ON BOULDER-CLAY The material known as _Boulder-Clay_ presents such distinctive features, and is so prevalent in our islands, that it deserves a few separate remarks. From a coating a foot or two in thickness, it swells in places to a hundred feet or more, and may form the important round-backed hills to which Maxwell Close reserved the name of _drumlins_. [Illustration: Fig. 11. Boulder-Clay, Crich, Derbyshire.] It consists essentially of mixed materials, unsifted by water, huge boulders of various rocks occurring side by side with angular fragments and pebbles of all sizes, set in a groundwork of loamy clay (Fig. 11). Sands and gravels are often associated with the boulder-clay, and result from the local washing of the mass in copious floods of water. The blocks are here on the whole more rounded, and the sandy part of the loam predominates. Blocks of shale and limestone, and even of sandstone and quartzite, occurring in the boulder-clay, bear the characteristic striations that we now recognise as due to glacial action. The sand and small stones have, in fact, been held against the larger ones by solid ice, and have cut and grooved their surfaces. Shales and schists have gone to pieces and have provided the clayey groundwork. The whole of the material has been at one time embedded in and moved forward by glacier-ice. [Illustration: Fig. 12. Arctic Glacier charged with stones and clay. Side of the Nordenskiöld Glacier, Billen Bay, Spitsbergen. The top of the ice appears in the left-hand upper corner of the picture.]