The article by Mr. Clifton Ward upon “Rock Fissures” in the GeologicalMagazine for June, has reminded me of a special characteristic of some igneous rocks, which has been perhaps as yet imperfectly appreciated. I mean by igneous rocks, such masses of subterranean, mineral matter as have apparently thrust themselves upwards to the surface of the globe by a process of internal expansion or intumescence, caused by heat —not always attaining absolute fusion, but in some degree owing to the general permeation of the matter by super-heated water, or steam, or other gases.