Summary: In this paper an endeavour has been made to establish the following points: That septarian structure consists of a polygonal system of cracks corresponding to a mud desiccation structure. That the cracking of the nodule is due to the desiccation of a colloidal centre by chemical means. That the nodules originated by the rhythmic precipitation of solutions diffusing through a colloid according to Liesegang's laws.

Finally, in addition to acknowledgements made in the text, I beg leave to thank Professors H. H. Swinnerton and C. H. Bulleid for help-ful criticism; Mr. H. Preston for placing his collection at my disposal; the chemical staff of University College, Nottingbam, for generous assistance; and Dr. G. T. Prior for his kind attention at the British Museum.