recognizes on sight the hard, smooth surfaces of water-worn rocks. They are exemplified in beach and river pebbles, in sea-worn cliffs, and where rivers flow over the solid ledges. The pot-hole (page 17) is a well-marked and specially interesting rock-form, due to current or river erosion. Ice has also left highly characteristic traces upon the rocks in all latitudes covered by the great ice-sheet. These consist chiefly of polished, grooved, and scratched or striated surfaces, the grooves and scratches showing the direction in which the ice moved. The organic agencies, as already noted, accomplish very little in the way of erosion, especially in the hard rocks, but the rock-borings made by certain mollusks and echinoderms may be mentioned as one unimportant but characteristic form due to organic erosion. ------------------------------------------------------------------------ APPENDIX The following collections are especially prepared and arranged for use with this text: _Weathering_ 1 Diabase *2 “ weathered *3 “ disintegrated °4 Felsite: Angular fragment °5 “ Water rounded pebble _Formation of Coals_ *6 Peat °7 Lignite 8 Bituminous *9 Cannel coal °10 Anthracite °11 Native coke _Rock-forming Minerals_ *12 Graphite °13 Halite *14 Limonite *15 Hematite *16 Magnetite 17 “ Lodestone *18 Quartz: Glassy 19 “ Flint 20 “ Chert 21 Opalized wood *22 Gypsum *23 Calcite °24 Dolomite 25 Siderite *26 Hornblende °27 Pyroxene *28 Muscovite 29 Biotite *30 Orthoclase °31 Albite *32 Labradorite *33 Kaolinite 34 Talc *35 Serpentine °36 Chlorite 37 Glauconite (Green Sand) 38 Chrysolite 39 Garnet 40 Pyrite _Sedimentary and Metamorphic Rocks_