Many clays give much trouble by expanding, or swelling out, when the excavation penetrates the layer, and although extra strong timbering 165 may be used, and be placed closer together, the logs and planks are frequently bulged out and broken by the action of the clay. Specially strong supports are required for this description of clay, and extra thickness of material in the permanent work of side walls and arching. Solid unbroken beds of chalk are not difficult to cut through: the material is easy to work, and the excavation will stand with ordinary timbering; but where the chalk is broken and intersected with deep pockets of gravel and sand, the operations are very much impeded. The loose material, once set free by cutting through the confining barrier of chalk, will quickly fall into and fill up the excavation if not held back by strong timbering. Side walls and arching are generally necessary for tunnels through chalk. Soft wet clay, quicksands, or other strata having springs of water percolating through them, are serious obstacles in the way of expeditious tunnelling. No sooner is one cube yard of this soft material removed than another slides down, or is washed down, to take its place. When once the excavation taps the water-bearing strata, large volumes of water will find their way into the workings, and must be conveyed away to the mouth of the tunnel, or pumped up through the nearest shaft. The timbering of the sides and roof through this description of working is very tedious, and attended also with a considerable amount of risk. The absence of really solid ground on which to place or shore up the supports, taxes the skill of the excavators, and very often, when a short length has been made apparently secure, it will come down with a run, compelling all hands to beat a hasty retreat. The permanent lining through such treacherous material should follow the excavation very closely, and special care should be exercised in building the walls, arching and invert. In the excavation through stratified rocks it is necessary to note carefully the lie of the strata, whether horizontal, vertical, or shelving, as with each one the excavators are exposed to risks, against which every precaution should be taken. A large horizontal slab of solid-looking rock will suddenly break and fall down without any warning. A heavy mass from a vertical layer, perhaps unkeyed, or loosened, by an adjacent blasting operation, drops down when least expected; and pieces from the high side of the shelving layers detach themselves and slide into the working in a most unaccountable manner.