POTHOLES. Cylindrical holes in rock ledges usually in or near beds of streams. They are caused by the wearing action of sand and pebbles whirled by the water. They have been called giant kettles, giant caldrons, In dian kettles, wells and the Spanish name, re molionos. Their size and character vary greatly, many of the larger ones being 10 feet in diameter and 25 feet deep. One in Saint Croix, Dallas, Wis., is 27 feet in diameter, another with diam eter of 15 feet is 65 feet deep, and one 12 feet in diameter is 160 feet deep. The pot-holes of the °Glacial Gardens° of Lucerne in Switzer land are famous. In some cases, notably on Kicldnghorse River, near Field, British Colum bia, a small natural bridge has resulted from the coalescence of the lower parts of two or more pot-holes. The canyon at Watkins Glen, N. Y., consists partly of coalescing pot-holes. Pot-holes develop only in streams that are eroding in fairly well consolidated rock. They evolve from small hollows due to solution, removal of a lense or pebble or a joint-crack. Such hollows are necessary to hold the first few pebbles which begin the grinding. Too much
sediment stops the process, but otherwise flood stages of the stream greatly facilitate the erosion. The nature and jointing of the rock and declivity and volume of the stream are im portant factors. Many holes, are of glacial origin. not caused by the ice, however, but by water falling over the edge of the ice or into holes extending •to the rock floor. Ir regular pot-holes or basins also occur along the coast, scoured out by tidal or other currents. Plunge pools are frequent at the foot of water falls and a notable one on the Canadian side of Niagara Falls according to J. W. Spencer is 72 feet deep. Small pits due to swirling of hard parts of sea urchins have been observed on sea coast in New Brunswick and at Biar ritz, France. Consult article by E. D. Elson (in Scientific Monthly, Vol. V, pp. 554-567; Vol. VI, pp. 37-51, 1917-18) ; Gilbert, G. K., Geological Society of America' (Vol. XVII pp. 317-370, 1906) ; Upham, W. (ib., Vol. XII, pp. 25-44, 1900).