In his book "Slate from Coniston", Alistair Cameron has a diagram showing three veins of slate stretching across the Lake District. The one that goes from Banniside to Brathay and finally to Longsleddale is said to be sedimentary slate, rather than volcanic. He says it is blue/black veined and didn't rive well so was used for flag stones. Here's the evidence. Now to look for the volcanic slates.
