120 



Subsurface Geologic Methods 



have been picked, attention should be directed to the character of the 

 sediments in each unit and to the nature of the transition between units. 

 Some of these characteristics can be determined by megascopic examina- 

 tion; others can be determined better with the binocular microscope. The 

 following discussion applies to both megascopic and binocular examina- 

 tion of samples. 



CLASTIC 



CRYSTALLINE 



PEBBLES 



GRANULES 



VERY COARSE 



COARSE 



MEDIUM 



FINE 



VERY FINE 



SILT 



CLAY 



10. 



.01 



.001 



COARSE 



3.2 



FINE 



COARSE 



56 



MEDIUM 



.32 



FINE 



18- 



VERY FINE 



COARSE 



096 



MEDIUM 



032 



FINE 



.018- 



VERY FINE 



COARSE 



.0096 - 



0032 



MEDIUM 



FINE 



.0018- 



VERY FINE 



VERY FINE 



CRYPT06RAINED 



MEGACRYSTALLINE 



.02 



COARSELY 

 CRYSTALLINE 



MEDIO - 

 CRYSTALLINE 



FINELY 

 CRYSTALLINE 



CRYPTO- 

 CRYSTALLINE 



MAT 

 ( INVISIBLE ) 



Figure 51. Suggested grade scales for clastic and crystalline sedimentary rocks. 



