Friends, this red rock is fragile and will break like any good china cup.  The formation process is similar to producing bricks or fine tiles.  A clay (bentonite) is baked by a coal vein that is burning underground.  The coal is a soft lignite and may be spontaneously ignited or the vein may be lit by grass fires whipped by the wind.  These fires may burn  trees and then travel down the roots to the seam underground.  When the various clays are baked and glazed by the igneous process, they are a form of porcelain or as it is locally called, scoria.  When a chunk of this light rock is dropped, it clinks on a hard surface, and it is often called North Dakota Clinker. I know i am at home when i see the scoria peaks at sunset.  til Tomorrow MJ