The fact that argillite continued in use until the very last does not affect this conclusion.
Next above the Altyn lies thirty-four hundred feet of Appekunny argillite, or dull-green shale.
It is very significant that we never find one where the workman used both flint and argillite.
Sandstone may be converted into quartzite, and shale into argillite, a compact, massive clay rock.
Here anthracitiferous coal is found in beds of argillite and sandstone.