not in very perfect crystals, though sometimes incomplete octagonal sections are visible; the tetragonal cleavage, strong double refraction and uniaxial interference figure distinguish them readily from other minerals. Commonly they weather to micaceous aggregate, but sometimes an isotropic substance of unknown nature is seen replacing them. In crystalline limestones and calc–silicate rocks they occur in small and usually inconspicuous grains mingled with the other components of the rock. Large, nearly idiomorphic crystals are sometimes found in argillaceous rocks (altered calcareous shales) that have suffered thermal metamorphism. In the Pyrenees there are extensive outcrops of limestone penetrated by igneous rocks described as ophites (varieties of diabase) and lherzolites (peridotites). At the contacts scapolite occurs in a great number of places, both in the limestones and in the calcareous shales that accompany them. In some of these rocks large crystals of one of the scapolite minerals (an inch or two in length) occur, usually as octagonal prisms with imperfect terminations. In others the mineral is found in small irregular grains. It is sometimes clear, but often crowded with minute enclosures of augite, tourmaline, biotite and other minerals, such as constitute the surrounding matrix. From these districts also a black variety is well known, filled with minute graphitic enclosures, often exceedingly small and rendering the mineral nearly opaque. The names couzeranite and dipyre are often given to this kind of scapolite. Apparently the presence of chlorine in small quantities, which may often be detected in limestones, to some extent determines the formation of the mineral. Mafic igneous rocks In many mafic igneous rocks, such as gabbro and diabase, scapolite replaces feldspar by a secondary or metasomatic process. Some Norwegian scapolite-gabbros (or diorite) examined microscopically furnish examples of every stage of the process. The chemical changes involved are really small, one of the most important being the assumption of a small amount of chlorine in the new molecule. Often the scapolite is seen spreading through the feldspar, portions being completely replaced, while others are still fresh and unaltered. The feldspar does not weather, but remains fresh, and the transformation resembles metamorphism rather than weathering. It is not a superficial process, but apparently takes place at some depth under pressure, and probably through the operation of solutions or vapours containing chlorides. The basic soda-lime feldspars (labradorite to anorthite) are those that undergo this type of alteration. Many instances of scapolitization have been described from the