138 Subsurface Geologic Methods 



sands on the two sides of such lines appear to have been derived from 

 different sources. Thus roundness has been used to outline petrographic 

 provinces. 



Because rounding of grains of very fine-sand size occurs very slowly, 

 the presence of rounded grains of that size in a sediment strongly suggests 

 the derivation of that sediment from a pre-existing sedimentary source. 

 Also the presence of rounded and angular grains of the same mineral in 

 a sample suggests derivation of the sample from two or more sources, one 



100% ROUND 



1 



100% z V V v A7 -v^iv^' ja r^'V* \/ AlOO% 



SUBANGULAR ANGULAR 



••MASSILON o SHARON ■<*. BLACK HAND 



(Salt) (Maxton) (Big Injun) 



Figure 57. Roundness of tourmalines in Massillon, Sharon, and Black Hand formations 



of Ohio (1/8-1/16 mm. size). 



of which is sedimentary. In figure 57 the Sharon is such a mixed sand. 

 The use of roundness has been discussed in more detail by Rittenhouse."*^ 



Minor Minerals 



Some sands contain small percentages of grains that are distinctive 

 in color or appearance. A group of sands in the same area or different 

 parts of a single sand may contain such distinctive grains in different pro- 



*^ Rittenhouse, Gordon, Grain Roundness — A Valuable Geologic Tool: Am. Assoc. Petroleum Geolo- 

 gists Bull., vol. 30, no. 7, pp. 1192-1197, July 1946. 



