Septaria

Septaria, concretionary nodules of clay-ironstone or impure limestone, in which the inner first-formed parts have contracted more than the outer, producing crossing series of cracks which have afterwards fined by infiltration with crystalline calcite. The clay-ironstone septaria occur in the shales of the Coal Measures, and the calcareous ones in many clays. Those in the London Clay, known to quarrymen as "turtle stones," are often of a grey earthy texture, traversed by lemon-yellow septa, and are cut and polished as table-tops. They range from six inches to several feet in diameter. They are largely collected in Sheppey and dredged up off Harwich for the manufacture of Roman cement.