William Smith's 1815 geologic map A Delineation of the Strata of England and Wales with Part of Scotland did not initially portray the stratum ‘Trap Rocks’. He did, however, include ‘Trap Rocks’ on the much simplified, reduced-scale 1820 map entitled A New Geological Map of England and Wales, a revision of his 1815 map. On the 1820 map, outcrop patterns in the Midland Valley resemble patterns seen on two earlier maps of Scotland; those by Louis-Albert Necker de Saussure in 1808 and Jean-François Berger in 1816. The present comparison examines all the changes Smith made in Scotland between his 1815 map and his 1820 map, and relates them to what is on the two earlier maps by Necker and Berger. What might not be known is how Smith learned of the maps by Necker and Berger and what they showed.

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