Ch. 33] CHARACTER AND FORMATION 623 



in carbonate rock. We should not lose sight of the possible value of 

 impurities to determine distribution and orientation, which may have 

 important implications in the development of porosity and permeabil- 

 ity. 



Clastic carbonates. There is growing recognition of the part that 

 clastic carbonates play in the composition of many limestones and 

 dolomites. The action of circulating ground waters and the resulting 

 development of porosity will obviously follow patterns in fragmental 

 rocks different from those in more uniform chemical or biochemical 

 precipitates. Petrofabric investigation could be expected to show re- 

 lationship of grain size to porosity, cementation, and other phenomena 

 related to the development of porosity and permeability. 



Character and Formation of Porosity and Permeability in Car- 

 bonate Rocks 



A broad investigation of the formation and development of porosity 

 and permeability will employ many of the foregoing techniques. Much 

 has been written about changes that take place when a limestone is 

 altered to dolomite or dolomitic limestone. More consideration should 

 be given to the development of porosity in dolomites deposited as dolo- 

 mites. A searching study of the paragenetic history of carbonate dep- 

 osition is an essential approach to the problem. Equally important is 

 a thorough investigation of the increase or destruction of porosity by 

 chemical solution or precipitation. 



Minor quantities of anhydrite, salt, and gypsum are found in fre- 

 quent association with carbonate rocks. Such minerals are dissolved 

 more readily, even in non-acidic waters, than are limestone and dolo- 

 mite. The resulting vug holes and ribbon-like channels provide ready 

 passage for ground waters to improve the existing porosity. 



The terms primary porosity and secondary porosity should be 

 avoided because of the difficulty of definition. It is generally recog- 

 nized that certain rocks are both porous and permeable when deposited 

 and remain so during and after lithification. More investigations 

 should be made of the changes that take place in original porosity and 

 permeability as a result of solution and deposition. 



Three methods are especially well-adapted to the study of porosity 

 formation and the geological processes involved. 



(1) Petrographic study of thin and polished sections and the use 

 of staining and etching techniques is a basic approach to the establish- 

 ment of the geologic history of changes that have taken place within 



