Decadocrinus grandis W. & Spr. Fig. 17. Plates of ventral sac with terminal spine, showing the pores at the sides of the plates, and also between the plates and the terminal spine..... Scaphiocrinus arboreus Worthen. Fig. 18. Entire specimen, showing upper part of perforated ventral sac, anterior view.... Fig. 20. Exposed part of the ventral sac shown in fig. 19, with Fig. 21. The upper part of ventral sac of another specimen........ Fig. 23. Part of the ventral sac of another specimen ground off to half the depth of the plates, showing perforation of the sac by pores at the middle of the sides of the plates.... } MINERALOGICAL AND PETROGRAPHIC STUDY OF THE GABBROID ROCKS OF MINNESOTA, AND MORE PARTICULARLY, OF Introduction. THE PLAGIOCLASYTES.* (Plates VIII to XIX.) By ALEXANDER N. WINCHELL, Butte, Mont. CONTENTS. 1. Geography and topography of the region studied. 2. Geographic distribution of the rock types. 3. Outline of the geology of the district according to the latest views. 4. Correlation of the nomenclatures. Chapter I. Olivine gabbro. 1. Geological occurrence. 2. Exterior characters. 3. Texture and mineralogical composition. 4. Detailed study of the minerals. 5. Inclusions. 6. Alterations. 7. Chemical composition. *The facts presented in this series of papers are essentially the same as those which appeared in the form of a thesis, presented to the Faculty of Sciences of Paris to obtain the title of Doctor of the University of Paris, and entitled: Etude minéralogique et petrographique des roches gabbroïques de l'Etat de Minnesota, Etats Unis, et plus specialement des anorthosites. Paris 1900. Chapter II. Normal diabase. 1. Geological occurrence. 2. Exterior characters. 3. Texture and mineralogical composition. 4. Chemical composition. Chapter III. Olivine diabase. 1. Geological occurrence. 2. Exterior characters. 3. Texture and mineralogical composition. 4. Detailed study of the minerals. 1). Abnormal pyroxene. Chapter IV. Plagioclasyte. 1. Geological occurrence. 2. Exterior characters. 3. Mineralogical composition, and order of crystallization. 4. Detailed study of the primary minerals. Chapter VII. Cordierite noryte. 1. Geological occurrence. 2. Exterior characters. 3. Texture and mineralogical composition. 4. Detailed study of the minerals. 1). Labradorite. 2). Bronzite and enstatite. 3). Anomite. 4). Cordierite. 5). Quartz. 6). Magnetite and pyrite. 7). Staurolite and graphite. 8). Zircon. 9). Apatite, epidote, and spinel. 10). Anthophyllite and muscovite. 5. Chemical composition. 6. Origin. Chapter VIII. Quartz gabbro. 1. Geological occurrence. 2. Exterior characters. 3. Texture and mineralogical composition. 4. Detailed study of minerals. 5. Chemical composition. 6. Origin. Chapter IX. Silicoferrolyte. 1. Geological occurrence. 2. Exterior characters. 3. Texture and mineralogical composition. 5. Chemical composition. 6. Origin. Chapter X. Résumé and conclusions. 1. Comparative petrographic study. 2). Modifications due to: a). Conditions of solidification. b). Original heterogeneity or differentiation of magma. c). Contacts (endomorphism and exomorphism). 3). Classification. 2. Comparative mineralogical study. 3. Comparative chemical study. 1). Methods of analyses. 2). Results of analyses. 3). Graphic representation of analyses. a). Method of Michel Lévy. b). Method of Brögger. c). Method of Becke. d). Method of Iddings. INTRODUCTION. The great continental nucleus, which Dana has called the "North American protaxis," extends, as is well known, from the eastern limits of Labrador to the head waters of the Mississippi river and from New York state to the Arctic islands of |