Samples obtained from the sea bottom in the vicinity of the islands of Capri and Sardinia have shown the following mineral composition of their clay fraction : illite, chlorite, kaolinite, and montmorillonite in a mixed-layer assemblage. These determinations have been made by X-ray diffraction. Quantitative estimates and pétrographic examinations show that the montmorillonite is of volcanic origin and is altered to produce mixed-layer structures. The alteration continues with the formation of illite from the mixed-layers. No variation in the abundance of kaolinite has been observed. The alterations mentioned above are functions of the thickness of the sediments.