FOREWORD.Pyrite and marcasite occur in many sedimentary beds and are common in slates, shales, mudstones or "foulbottoms." Syngenetic deposition of FeS 2 seems highly probable, provided that the sulphides are disseminated throughout these rocks. Sulphate-reducing, and hence H 2 S-producing, bacteria may ultimately be responsible for the precipitation of the primary sulphides. Most investigators will be convinced of this mode of origin or at least will accept this possibility.However it is not my intention to discuss this point here. I think pyrite and marcasite, if dispersed throughout the rocks mentioned above, may have been formed syngenetically with the sediments, and the precipitation of the iron sulphides may have been largely governed by H 2 S produced by sulphate-reducing bacteria. Furthermore, I am convinced of the important changes, such as deformation, recrystallization and redeposition, which may result from different kinds of metamorphism. Diagenesis followed by intense metamorphism is likely to render such deposits almost unrecognizable.The writer also believes that some copper and lead sulphides such as those in the Red Beds, and compounds of Va, U and other elements may be concentrated in sediments by descending solutions. Usually however these deposits are supposed to be epigenetic, since they exhibit widespread replacement. These occurrences are mainly limited to shallow waterbeds on the flanks of continental areas.On the other hand, especially in German literature, some examples have been advanced of large and massive pyritic bodies, containing only minor gangue or country rock, and have been classified as sedimentary and syngenetic with the original material of the slates or shales.Schneiderhoehn, 1 who is convinced of the syngetic origin of these pyritic ore bodies, and others have published some articles in support of this view. One of Schneiderhoehn's arguments in favor of a syngenetic origin is the existince in these deposits of very small sulphide spheres composed of tightly packed grains which he thinks originally were sulphur bacteria. In one article on the copper shale of Mans feld Schneiderhoehn attempts to prove that the spheres are fossil organisms.In an earlier publication 2 the present writer strongly opposed this point of view. It seems desirable now to advance additional evidence, especially since Schneiderhoehn's viewpoints appear to have gained ground in European literature, in order to show that the validity of his assertions is open to question. As I see it, his interpretation cannot be used as definite criteria against an epigenetic origin.The following points of Schneiderhoehn's articles will be discussed: (1) Morphology of the sulphur bacteria; (2) Physiology and ecology of1 H. Schneiderhohn, Neues Jahrb, f. Min. G. P., 1923, B. B. 47, p. 1. Metall tmd Erz.1926.2 C. Schouten, Metasomatische Probleme, 1937. these organisms; (3) Chemistry of the various deposits; (4) Ore genetics, especially respecting the copper shale.On the first two points, reliance must be placed on data from the literature about recent forms of sulphur bacteria. The fourth point will be discussed in connection with my own ore microscopic observations.