sentences:  At the Silurian/Devonian boundary black cherts (locally called "lydites" or flinty slates) developed from radiolarians mainly in the Frankenwald region and in the Vogtland in Germany.Of great importance are the novaculites from Arkansas, Oklahoma and Texas which were deposited at the close of the Devonian.The novaculites are milky-white, thinly-bedded cherts of great hardness; they underwent a low-grade metamorphism during the Ouachita orogeny.