There are two sorts of slate in the Geological Garden, Welsh roofing slates and Cornish Delabole flooring slates. The pyramid roof of Marylebone Lower House is covered with Green Cumberland Slates, a third type.

Any fine-grained material can be converted into 'slate' by pressure. Welsh Slate (Blue­grey here) was originally a fine-grained mud, its' flat platy minerals becoming squeezed into parallel sheets by the pressure of mountain building. The parallel sheets are what are called cleavage planes which allow the slab to be split easily into thin roofing slates. Lake District Green Slate is actually a fine volcanic ash which has had its' platy miner­als compressed so that it too can be cleaved. It cannot be so thinly split as Welsh Slates as the mineral grains were coarser. By looking closely at split slates it is sometimes possible to see where the slate was struck from the block. This is called the percussion point and fracture lines radiate from it. The slate path shows some areas of white colour. These are probably Calcite veins and were formed in the original slate. Other marks are formed, unfortunately, by the mortar left on the surface of the slate during the building. It is hoped that this will wear away, but it will take a long time.

One of the Cornish Delabole Slate flooring slabs showing the percussion point X and the radiating lines of cleavage. In 'One Man's England', by W.G.Hoskins, there is a photograph of the enormous quarry at Delabole where slate has been extracted for 400 years.

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Pages 20-21

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