James Hutton's scondary sandstone strata now called Upper Old Red Sandstone

Sandstone is a detrital sedimentary rock consisting of individual grains of sand-size particles 0.06 to 2 millileters in diameter either set in a fine-grained matrix (silt or clay) or bonded by chemical cement [1].
Upper Old Red Sandstone is found as surface rock in Scotland along the North Sea coast, where it helped James Hutton in the 1780s demonstrate his theory of geologic formations via cyclic processes occurring over long time spans [2]. Hutton called the rock system “secondary sandstone strata.” This coarse red sandstone is younger than the primary micaceous schistus (Silurian graywacke), the other distinct type of surface rock at Hutton's outcrop location, thought to have been formed separately (the secondary rock type after the primary one) at different geological times in erosion-sedimentation-uplift cycles.

Keywords: mineralogy, geology, history, Scotland, rock names

References
[1] Dictionary of Geology and Mineralogy. Second Edition. McGraw-Hill, New York, 2003.
[2] Jack Repcheck: The Man Who Found Time • James Hutton and the Discovery of the Earth's Antiquity. Persus Publishing, Cambridge, MA, 2003.