. Bulletin of the Geological Society of America. Geology. 464 A. A. JULIEX AMPHIBOLE SCHISTS OF MANHATTAN ISLAND complement of the angle required for inclination of the proper cross-sec- tion toward the tibration. From a thin-section so prepared the actual areas (a) of the different minerals are determined by means of photomi- crographs or of drawings, and their proportionate volumes estimated as before. It is apparent, however, that such a normal cross-section in longitudinal direction of a blade can possess sufficient area only in prisms or blades of a certain v %> ^ -^ % Figu
. Bulletin of the Geological Society of America. Geology. 464 A. A. JULIEX AMPHIBOLE SCHISTS OF MANHATTAN ISLAND complement of the angle required for inclination of the proper cross-sec- tion toward the tibration. From a thin-section so prepared the actual areas (a) of the different minerals are determined by means of photomi- crographs or of drawings, and their proportionate volumes estimated as before. It is apparent, however, that such a normal cross-section in longitudinal direction of a blade can possess sufficient area only in prisms or blades of a certain v %> ^ -^ % Figure b.—Diagram showing proper Section Plane inclined lo schist Plane, in thin Blades. C. Schists of irregular texture, made up of thin blades of the predom- inant mineral, with the other minerals in intervening laminse of unequal or greater thickness. In such a rock the section plane should lie with its axis in direction of the fibration, inclmed at a certain angle to the schist plane, as illustrated in figure 5. In the small terminal right-angled triangle the desired angle A' (=^ A) may be calculated from the known factors, t (thickness of the blade), assumed to form the base of the triangle, (1/2 =^—r J , as- sumed to form the hypotenuse, by the formula sin A' = -jr. From a thin-section sliced at that angle toward the schist plane, photo- micrographs are taken, and by means of photographic prints or of draw- ings, as before, the actual areas (a) of all the mineral plates are directly determined and tlie proportionate mineral composition of the schist thereby calculated. Sixth, a. There still remains another class of rocks, chiefly coarse schists, whose folia may lie in all positions, without parallelism of either their planes or their axes toward each other or toward the folia- tion plane. According to their mode of distribution the texture of the rock may be uniform or irregular, with more or less imperfect schistose structure. Their treatment may be a matter of judgment
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1890
