I have been investigating the cellar in our house (approximately 150 years old in parts) and in the cellar, between the slate rocks which make up the walls there is a clay like structure which I guess holds things together. My question is, would it have been clay originally, or is it cement which has gone off over the years - there is quite a lot of water in there! If it was cement, any great ideas about how I clean out the clay and replace it? I guess a large syringe type implement would be ideal for putting mortar between the rocks, but don't know if such a thing exists. All suggestions appreciated!!