Microscopic observation is valuable for elucidating diagenetic phenomena in clays subsequent to deposition. Examples:—

1. (1) Pressure can enlarge the regions of parallel orientation in kaolinite (Fig. 1), even giving zoned regions (Figs. 2, 3).

2. (2) Solution and re-precipitation may give vermicular kaolinite (Figs. 4,5), “fluidic” halloysite (Fig. 6), or vermicular dickite on nuclei of hydrargillite (Fig. 7).

3. (3) Montmorillonite may form from halloysite by action of basic siliceous waters (Fig. 8); or (together with halloysite) from kaolinite (Fig. 9).

4. (4) Illite may recrystallise, giving large hexagonal forms (Fig. 10).