in Venezuela, in which visual estimation is made, and each mineral is assigned 

 a frequency number based upon its relative abundance, as shown below. 



Frequency Number Descriptive Term 



5 Flood 



4 Abundant 



3 Common 



2 Fairly common 



1 Rare 



Very rare 



These numbers are only symbols, not true numbers in a mathematical sense. 



When time is available and an accurate grain count is made, a mechanical 

 stage is attached to the microscope so that spaced traverses may be made across 

 the slide. Dryden (1931) has pointed out that it is not profitable to count all 

 grains, as the law of diminishing returns is involved. The increased accuracy 

 obtained after counting approximately 300 grains does not justify additional 

 counting. Percentages should be calculated to the nearest whole number. 



After the relative mineral frequencies have been determined, the data 

 usually are presented in graphical form. Histograms, bar diagrams, profile charts, 

 star diagrams, and tables of various types have been used. Krumbein and 

 Pettijohn (1938) and Feo-Codecido (1956) give examples of such tabulations 

 and diagrams. 



interpretation of Data 



Interpretation of heavy-mineral data has received much attention by sedi- 

 mentary petrologists, and differences of opinion naturally exist. Heavy-mineral 

 studies have been used principally in determining the source rocks or provenance 

 of sediments and in correlating and identifying stratigraphic units. 



Pettijohn (1957) gives an excellent review and discussion of source 

 provenance and the effects of transportation or dispersal of detrital minerals. 

 One of the problems of heavy-mineral studies is the variation of complexity of 

 mineral suites with the geologic age of the sediment. The older Paleozoic rocks 

 usually contain a simple suite of the more resistant minerals such as zircon, 

 tourmaline, rutile, staurolite, and garnet. Younger rocks, particularly the 

 Recent deposits, usually contain more diversified mineral species. In addition 

 to the more resistant minerals listed above, the younger rocks often contain 

 pyroxenes, amphiboles, and other less resistant minerals such as epidote, kyanite, 

 and sphene. Pettijohn (1941) believes that in the older rocks the less resistant 

 minerals have been removed by intrastratal solution, whereas, Krynine (1946) 

 believes that the variation in mineral suites are due principally to differences 



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