They are always formed of argillite.
"The Prehistoric World" by E. A. Allen
Slate is, as we know, merely a variety of argillite.
"Scientific American Supplement, No. 1157, March 5, 1898" by Various
Next above the Altyn lies thirty-four hundred feet of Appekunny argillite, or dull-green shale.
"The Book of the National Parks" by Robert Sterling Yard
Near the upper end of the camp, Avalanche Creek has cut a deep, narrow gorge through brilliant red argillite.
"Glacier National Park [Montana]" by United States Dept. of the Interior
Haida Argillite Figure, ca 1860 PBS.
It turns out arrowheads can be made of argillite, chalcedony, chert/flint, diorite, hematite, jasper, rhyolite, siltstone, crystal quartz, quartz, and quartzite.
ARGILLITE — Until the last few years of his life, Bill Kelly's best friends rarely ever heard him talk about being a soldier during the Battle of the Bulge .
ARGILLITE — A stalled proposal to expand horse trails at Greenbo Lake State Resort Park may get under way soon thanks to a donation by a local industry.