Ordnance Survey Office,
April 9th 1830
An uniform System of Indicoclature is expedient in the Geological Plans and Sections connected with the Ordnance Survey; and I therefore direct your attention to the following Table of Letters and Colors by which the Rocks and Strata of most ordinary occurrence will be expressed.
I am, Sir,
Your very obedient Servant,
Lieutenant-Colonel Royal Engineers,
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