130 Subsurface Geologic Methods 



determined from a wet or a dry sample. For binocular examination of 

 both wet and dry samples magnifications of 12 to 24 diameters are com- 

 monly used. Higher magnification may be used for special purposes. 

 For most low-power study, flourescent is superior to incandescent lighting. 

 The percentage of various constituents is usually estimated. 



Hills ^'^ notes that there are two principal ways of describing samples: 



. . . The first of these is the interpretative system, in which the geologist 

 picks out the cuttings which he believes to be representative of the formations 

 penetrated and describes the entire sample as composed of this rock The rest 

 of the sample is assumed to be cavings. This kind of description brings out 

 formational changes and is of greatest value in areas where the various for- 

 mations are of wide extent and relatively constant character, as in the Paleo- 

 zoic of the Midcontinent region. In areas of rapid lateral gradation in the 

 lithologic character of formations, as in the Permian Basin of west Texas, this 

 method results in masking of lateral variations and misinterpretations of the 

 nature of the stratigraphic column. This, of course, results in miscorrelation 

 of the well logs. 



In regions of pronounced lateral gradation, it has been found that a second 

 method of sample description is most satisfactory. This is the percentage de- 

 scription, where the geologist describes all material in the sample, disregard- 

 ing obvious foreign substances and cavings. This system, though making it 

 difficult to determine formational boundaries from the sample log, shows the 

 gradations of the beds and often enables one to trace a horizon through differ- 

 ent sedimentary facies. 



Hills also discusses many other points on sample examination that 

 could not be covered in this section. 



Converted Binocular Microscope 



Recently the writer has used two polaroid plates to convert a binocu- 

 lar microscope into a low-power polarizing microscope. The maximum 

 magnification that can be obtained is low, being essentially the same 

 as the low power of a petrographic microscope. This permits study of 

 sand but not of silt and clay. The writer first tried the conversion by 

 mounting sand grains in clove oil on one lens of a pair of clip-on polaroid 

 sunglasses, placing the other lens above and at right angles to the first, 

 and fastening the two lenses in place with drafting tape. Light was trans- 

 mitted from below the glass stage of the microscope, and the grains were 

 moved by pressing on the upper lens. In fine and medium sands, quartz 

 grains, which are single crystals, were differentiated from chert grains 

 and rock fragments, which are composed of many smaller crystals. Micro- 

 cline and plagioclase feldspars could be recognized by their twinning. 

 The high birefringence and relief of carbonates separated them from 

 quartz, feldspar, and chert. It should be possible to differentiate anhy- 

 drite, which has strong birefringence (0.044) and an index of refraction 

 higher than clove oil, from gypsum, which has weak birefringence (0.010 

 — quartz is 0.009) and an index of refraction lower than clove oil. 



« Hills, J. M., op. cit. 



