Fig. 13From: K-feldspar rich shales from Jurassic bedded cherts in southeastern SloveniaAl2O3 versus SiO2 diagram showing the biogenic origin of silica in cherts, and terrigenous origin in shales. NASC North American shale composite, PAAS post-Achaean Australian Shale. Symbols same as for Fig. 4 Back to article page