Webster's Revised Unabridged Dictionary
G. grauwacke,; grau, gray + wacke, wacke. See Gray, and Wacke, and cf. Grauwacke
Chambers's Twentieth Century Dictionary
Ger. grauwacke—grau, gray, wacke, a flint.
Is it primitive, or is it graywacke like Catskill Mountains?
"Personal Memoirs Of A Residence Of Thirty Years With The Indian Tribes On The American Frontiers" by Henry Rowe Schoolcraft
It is a boulder of fire graywack, twelve feet long and five feet high, and faces the bed of the river.
"The Pre-Columbian Discovery of America by the Northmen" by B. F. De Costa