Subsurface Laboratory Methods . 165 



tory techniques of determining microfabric and the methods of its pres- 

 entation: "Laboratory Technique of Petrofabric Analysis," by Ingerson,^^ 

 which begins with the study of the hand specimen after it has been oriented 

 in the field and carries the reader through the steps of microscopic analysis 

 of the specimen and recording and presentation of the data; "Structural 

 Petrology of Deformed Rocks," by Fairbairn,^^ chapters 8, 9, and 10, giv- 

 ing much the same information as Ingerson's paper but with a somewhat 

 different presentation; and "Federow Method (Universal-Stage) of In- 

 dicatrix Orientation," by Haff,^^ a paper presenting the universal-stage 

 and its operation. 



In the laboratory the oriented hand specimens collected as described 

 above are studied megascopically or with the binocular or by both methods 

 to determine the selection of coordinates. The most prominent structure 

 plane such as schistosity is taken as the a-6 plane. The direction for the 

 thin sections are determined by the choice of the three reference axes. 

 Generally three sections are cut normal to a, b, and c (fig. 61). If there 

 is doubt as to the selection of the axes, random thin sections may be cut 

 and studied in order to discover any preferred orientation. 



Quartz, calcite, and the micas are the minerals commonly used for 

 the construction of orientation diagrams. In quartz the attitude of the 

 optic axis is determined; in calcite, the optic axis or the poles (the normal 

 to) of twin or glide planes are mapped; in the micas the poles of the 

 cleavage (010-plane) planes are plotted. The measurements of these 

 features are carried out on the universal stage mounted on a petrographic 

 microscope; the results are plotted on a Schmidt net (fig. 68). This net is 

 the equal-area, azimuth projection of the lower half of a sphere. 



If quartz orientations are being studied, the direction of the c-axis 

 is determined on the universal-stage. This axis is brought into coincidence 

 with the microscope axis; its original attitude may then be read from 

 the graduated circles of the U-stage. These values are a bearing and an 

 angle of inclination, which are then plotted on the net. Both are repre- 

 sented by a point on the diagram that in a three-dimensional solid would 

 be the point at which the c axis pierces the lower hemisphere. If mica 

 books are to be oriented, however, the cleavage planes are brought into 

 line with the plane containing the microscope axis, and the attitude of 

 the poles (the normals to) of the cleavage planes is plotted in much the 

 same manner as the quartz c axes. 



^° Ingerson, ilarl, Laboratory Technique of Petrofabric Analysis (pt. 2 of Structural Petrology by 

 Knopf, E. B., and Ingerson, Earl) : Geol. Soc. America Mem. 6, pp. 209-262, 1938. (A separate of this 

 is Carnegie Inst. Washington, Geophysics Lab., Paper 959.) 



" Fairbaim, H. W., Structural Petrology of Deformed Rocks, pp. 106-131, Cambridge, Mass., Addison- 

 Wesley Press, Inc., 1942. 



^' Haff, J. C, Federow Method (Universal-Stage) of Indicatrix Orientation: Colorado School of 

 Mines Quart., vol. 37, no. 3, pp. 3-28, July 1942. 



