Subsurface Laboratory Methods 139 



portions. Quantitative measurement of these differences may provide use- 

 ful criteria for correlation or differentation. Because distinctive grains 

 may be present in a ratio to the total of 1 to 1,000, 1 to 100,000, or even 

 less, the determination of the proportion by count would be very tedious 

 and time-consuming. By using a good sample splitter and a binocular 

 microscope, however, the relative proportions can be determined much 

 more rapidly. The procedure is as follows: 



The sample is disaggregated and sieved to size the sand. Sizing is 

 necessary because the proportion of distinctive grains may differ with 

 size, and different ratios would occur in coarse and fine sands. The sieve 

 separate is weighed to the nearest 0.01 gram on a triple-beam balance. 

 Using an Otto microsplit a random sample is split out. The number of 

 splits necessary to obtain the test sample is recorded; that is, the test 

 sample is 1/64, 1/128, or other fraction of the original weighed-sieve 

 separate. 



A 5- by 8-inch file card is folded into the shape of an M and the test 

 sample is scattered as evenly as possible along the trough of the M to 

 form a line of grains. The card is placed under the microscope, and the 

 number of distinctive grains in the test sample is recorded as the card 

 is moved across the microscope stage. Only distinctive grains are counted; 

 the others are ignored. The number of distinctive grains is calculated. 

 This calculation can be shown better by example than by a word descrip- 

 tion; for example, the sieve separate weighs 6.02 grams, the test sample 

 represents 1/256 of the sieve separate, and 17 of the distinctive grains 

 were in the test sample. The number of distinctive grains per gram is 



17x256 



— — — - — =723. It should be noted that in this example each distinctive 

 6.02 



grain recognized in the test sample represents 43 grains per gram. Con- 

 sequently, a difference of at least 600 grains per gram more or 400 grains 

 per gram less would be necessary to be considered significant. If twice as 

 large a sample had been counted and twice as many distinctive grains 

 had been recognized, differences of about 400 grains more or 300 grains 

 less would be considered significant. 



One requisite of the distinctive grains is that they must be approxi- 

 mately the same specific gravity and shape as the quartz grains. They 

 cannot be flaky grains like the micas or heavy minerals like the tourma- 

 lines or garnets. In the Appalachian Basin a variety of quartz that con- 

 tained small wormlike inclusions of green chlorite gave useful information. 



Thin Sections 



Thin sections can be used to very good advantage in studying detrital 

 mineralogy. They have the following advantages: (1) Much higher mag- 

 nifications are possible than with a binocular microscope, thus permitting 

 clear inspection of the smaller features of the rock. (2) The minerals 

 may be identified by means of their optical properties. (3) The sections, 



