128 



Subsurface Geologic Methods 



Texture may be indicated in part by the rock name and in part by 

 modifiers of the rock name. Thus the term "breccia" implies angularity 

 of the component grains, whereas the term "sand" has no roundness impli- 

 cation but may be modified by such terms as "angular" or "subangular." 



Structure 



Structure is somewhat similar to texture in meaning but is applied 

 to such large-scale characteristics of the rock as bedding, cross-bedding, 



CLAY 



(SERICITE a CHLORITE) 



FELDSPAR 



QUARTZ 



^AND CHERT) 

 Figure 54. Classification of sandstones. (Modified from Pettijohn.) 



jointing, and folding, which ordinarily are seen best in outcrops but may 

 be observed in some cores or hand specimens. Except for lamination, 

 structures seldom can be identified in drill cuttings. 



Rock Types 



No standard classification of rocks has been generally accepted for 

 megascopic and binocular subsurface work. A relatively simple system 

 that meets the needs of most subsurface work and follows the general 

 usage of many subsurface men is that shown in figure 53. Subdivision of 



