1132 



Subsurface Geologic Methods 



of these clays can be determined quickly by petrographic and X-ray-dif- 

 fraction analysis. If the number of samples available for test is so great 

 as to preclude testing of all, petrographic examination to select specimens 

 representative of the group frequently will simplify the test program with- 

 out sacrificing the significance of the results. 



Engineering petrography and geology in coordination serve to relate 

 the properties of the individual specimens subjected to laboratory tests to 



Figure 595. Petrographic examination of thin slice of metamorphic rock shown in 

 upper photograph (X50) revealed that mineral grains were arranged in parallel 

 planes and led to prediction that rock would fail along these planes. Crushing 

 tests conducted on cores of this rock (lower photograph) confirmed this predic- 

 tion. Line marked S on side of cores was predicted line of failure. 



