Primary micaceous schistus, today know as Silurian graywacke

Graywacke is an argillaceous sandstone characterized by an abundance of unstable mineral and rock fragments and a fine-grained clay matrix binding the larger sand-size detrital fragments [1].
Silurian graywacke formed during the Silurian period in the Paleozoic era, sometime between 430 and 390 million years ago. This type of shale is found as surface rock, for example, in Scotland along the North Sea coast, where it helped James Hutton in the 1780s demonstrate his theory of geologic formations via cycles of uplift, erosion and sedimentation occurring over long time spans [2]. Mineralogists at that time labeled this smooth, grayish stone as “primary micaceous schistus.”

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.