134 



Subsurface Geologic Methods 



by one or two rapid microscopic tests. With these criteria 90 percent 

 or more of most mineral suites can be identified by observation in plane- 

 polarized light. The speed of counting the grains on a slide may be 

 doubled or trebled and the eyestrain greatly reduced, if an assistant re- 

 cords the grain counts as they are made by the observer. 



Use of Heavy-Mineral Data 



The type of heavy-mineral data that should be obtained depends 

 primarily on the objectives of the investigation, the samples that are 

 available, and the heavy-mineral content of the samples. The heavy- 

 mineral distribution in sandstones is closely related to the size distribu- 

 tion of the light minerals in the sample and consequently the proportion 

 of heavy minerals, and sometimes their presence or absence in a particular 

 sample depends on the grain size of the sample. Some heavy minerals 

 are removed by solution and others are deposited after deposition. These 

 factors must be considered in determining the type of data to be obtained 

 and how the data should be interpreted. 



Ordinarily, if different kinds of heavy minerals occur in two samples, 

 for example a hornblende-epidote-ilmenite suite in one and a staurolite- 

 kyanite-magnetite suite in another, the samples reflect different sources for 

 the two samples. If the same kind of heavy minerals are present in two 

 samples but they are present in different proportions, different sources are 

 not indicated unless all of the minerals are of about the same specific 

 gravity. Major differences in ratios between varieties of the same mineral 

 or between minerals of about the same specific gravity usually indicate 

 different sources. Even in the last case, authigenic minerals must not be 

 used, and the possibility that some minerals may be removed by solution 

 must be given consideration. Pettijohn^^ and Dryden and Dry den '^^ give 

 the order of stability of minerals in sediments (both chemically and 

 mechanically weathered) and weathered rock respectively as follows: 



*^ Fettijohn, F. J., Persistence of Heavy Minerals and Geologic Age: Jour. Geology, vol. 49, no. 6, 

 pp. 610-625, Aug.-Sept. 1941. 



*' Dryden, Lincoln, and Dryden, Clarissa, Comparative Rates of W eathering of Some Common Heavy 

 Minerals: Jour. Sedimentary Petrography, vol. 16, pp. 91-96, 1946. 



