Four years ago I had the pleasure of reading a paper before this Society on the Erratic Boulders in the Valley of the Calder. Since then, extended observations have led to a confirmation of the views I then laid before the Society. The result of those observations forms the subject of this communication. It may assist a proper understanding of the subject if I recapitulate, as briefly as possible, the main facts of the case. The river has its source in two or three small streams which rise in the hills on the Lancashire side of the Pennine Anticlinal. These are joined into one stream, and pass along a narrow but deep valley cut at right angles to the range of elevated gritstone hills which form the boundary between Lancashire and Yorkshire. For five or six miles the valley is rarely more than about 200 yards in breadth, and on each side the slopes of the hills are extremely steep, composed of shales, surmounted by a precipitous gritstone escarpment. At Hebden Bridge the Calder is joined by the Hebden; in its course south-eastwards the valley gradually assumes larger dimensions, and south of Halifax spreads out into extensive level plains, along which the Calder has carved its channel with many devious turnings. On reaching the district from Mirfield to Wakefield these characteristics are still more apparent, its path being amongst the softer beds of the Coal-measures. Below Wakefield to the confluence of the Calder with the Aire at Castleford, the country …