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 thickness. 



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 as to closest 

 api)roximation in texture to one of the preceding classes ; otherwise the 

 plan suggested for A of the fifth class becomes pertinent. 



T.o recapitulate, the following is the procedure recommended : 



