much difficulty, arkose and altered granitic rock are difficult and sometimes im- 

 possible to distinguish even in thin section. Many arkoses have a vague clastic 

 texture that can be recognized, whereas granitic rock, though many of the grains 

 may be considerably altered, still retains its crystalline texture. 



Locally, oil reservoirs occur in basement rocks. Some of these, such as those 

 in weathered zones, have a general equidimensional distribution. Others, such as 

 those in shear zones or in certain beds of metasediments, have a linear distribu- 

 tion. The prospects for additional locations in either of the linear reservoirs can 

 be improved greatly by attempting to determine the trend of the structure control- 

 ling the reservoir. This evaluation can be improved by petrofabric analysis of 

 oriented cores. In addition, study of the type and distribution of basement rocks 

 may yield valuable information about subsurface structure. 



OIL-IMMERSION METHOD The best procedure for mineral identification 

 OF MINERAL is the oil-immersion method. The main ad- 



IDENTIFICATION vantages of this method are (1) preparation 



of a thin section is unnecessary; (2) deter- 

 mination of refractive indices is possible; and (3) relationships between optical 

 properties and physical properties, especially cleavage, can be determined. The 

 main disadvantage is that grain-to-grain relationships and texture cannot be de- 

 termined. For this reason, the oil-immersion method cannot totally replace the 

 thin-section techniques. The method consists essentially of immersing crushed 

 mineral grains in oils of known refractive index. Most of the optical properties 

 are determined as they are in thin section, and only the oils are needed in addi- 

 tion to standard petrographic equipment. 



Although cleavage aids in identifying minerals, it presents problems in 

 measuring principal indices of refraction. For example, rhombohedral cleavage 

 causes calcite grains to lie with their c-axes inclined to the stage at a high angle. 

 The extraordinary index can be determined only when the c-axis is horizontal; 

 therefore it is necessary to tilt the grain to the proper lie in some way. A uni- 

 versal stage can tilt the slide. This tilting can also be accomplished by propping 

 up a grain with surrounding smaller grains; or, if the inclination of the c-axis 

 can be determined from an interference figure, the extraordinary index can be 

 calculated from the measured partial extraordinary index. Biaxial minerals can 

 be handled in the same manner. For many common minerals having good 

 cleavage, such as the rhombohedral carbonates and plagioclase feldspars, values 

 for the indices on cleavage fragments are available (Winchell, 1951; Taylor, 

 1948) . 



The oil-immersion method should not be discounted simply because index 

 oils are not available. This method has other important advantages than index 

 determination. Cleavage and its relationship to optical elements can be deter- 



72 



