greywacke

In Cornwall and Devonshire, the lead ore is found in veins in killas, a clay-slate passing into greywacke.

The date assigned to its upheaval is "after the unfossiliferous greywacke, or Cambrian strata, and before the Silurian."

This fossil-bearing set of strata he first styled the “fossiliferous greywacke series,” changing it in 1835 to “Silurian system.”