As has been previously stated, all the phenomena that accompany spherulitic structure on an ordinary scale appear reproduced among lavas that may be called coarsely spherulitic. Ill-defined segregations, differing little from the matrix, the fluidal and banded structures of the rock running indifferently through both, are found measuring an inch or two in diameter, equally with forms of marked radial or concentric character. The scale on which certain processes of decomposition and reconstruction, especially those that affect differently the spherulite and the matrix, are revealed in these bold examples, and the interest attaching to their effect upon the ultimate character of the mass, have led me to bring together a few observations on the alteration of coarsely spherulitic rocks. Some of the finest instances, moreover, may be found in our own British lava-flows of Palæozoic age, and with these the field-worker is not unfrequently called upon to deal. The well-known black pitehstone of Planitz, near Zwickau in Saxony, which is of Permian sge, contains angular aggregates of calcite and chalcedony enclosed in large brown nodlfles. These remarkable occurrences were referred to by Cotta as early as 1849†, and his figure was copied by Delesse in a comprehensive paper on “Les Roches globuleuses”‡ Cotta considered the nodules surrounding the chalcedony as inclusions of an adjoining porphyry, like those in the pitehstone of Spechthausen ; but the microscope proves them to be in reality large brown spherulites, often 3 centims, in diameter, and differing only slightly from the glass. Why the agents of