and are higher still in Dupitila Group sandstones (82–
85 wt.%). Average SiO2 abundances in the Sylhet mudrocks are
characteristically less (b∼70%) than those in the sandstones (Table 2).
In both sandstones and mudrocks, SiO2 contents in the Tipam and
Dupitila Groups are greater at a given Al2O3than in the Barail and Surma
Groups; this is also the case for the Jaintia mudrocks (Fig. 3a)