Definition of Wacke:

part of speech: noun

A German miner's term for a soft earthy variety of trap- rock of a greyish- green colour, resembling indurated clay, and readily crumbling down when exposed to the weather.

Usage examples:

• The newest of these, connected with the higher series of coal- measures, is well exhibited along the shores of the Forth, in Fifeshire, where they consist of basalt with olivine, amygdaloid, greenstone, wacke and tuff. Sir Charles Lyell in "The Student's Elements of Geology".

• A beautiful bust of gray- wacke in our possession probably represents the same king. Georg Ebers in "The Complete Historical Romances of Georg Ebers".