Ch. 33] DYNAMIC PRINCIPLES INVOLVED 619 



Openings of a primary character are voids between individual crys- 

 tals and between cleavage planes of crystals. Certain sedimentary 

 rocks such as breccias, conglomerates, coquinas, and oolitic rocks have 

 primary porosity owing to the character of the material deposited. 

 Differences in crystal size and arrangement along bedding planes may 

 result in another type of primary openings. Earth movements result- 

 ing in faults, fractures, and joints produce still another type of void 

 which is considered primary in origin. 



The examination of a piece of porous limestone or dolomite seldom 

 permits reading of its geological history. This is due to its having 

 been subjected many times to agents possessing the power to alter the 

 porosity by solution or deposition. The basic factor to be held in 

 mind is that porosities as we now find them are undoubtedly the re- 

 sults of secondary alteration of one or more of the types of recognized 

 original porosity. Joints, fractures, and bedding planes furnish chan- 

 nels that make it possible for ground water to move rapidly, bringing 

 about such alteration as it is capable of rendering. 



Secondary porosity is thought to be formed mainly by the dissolving 

 power of connate or meteoric waters containing small concentrations 

 of carbonic, organic, or sulphuric acid. Such waters have an amazing 

 power to dissolve calcareous rocks if the movement of the water is 

 sufficient to keep the water undersaturated. It is believed that most 

 of the solution takes place above the ground-water table, where solu- 

 tions tend to be acid in character. Below the ground-water table, 

 solutions are usually more alkaline and become more nearly saturated; 

 therefore there is a normal tendency for carbonate precipitation at this 

 horizon. On the other hand, if the ground water moves with sufficient 

 rapidity, it may reach the water-table level with adequate dissolving 

 power to continue its work as it moves laterally. Interbedded shales or 

 those adjacent to carbonate rocks often contain considerable quan- 

 tities of carbonaceous material, both animal and vegetable, which was 

 not entirely oxidized before burial. This material in its present state 

 of preservation undoubtedly involved a release of considerable car- 

 bon dioxide which was available to combine and form carbonic acid. 

 By this process, solution of carbonate rocks could be accomplished well 

 below the ground-water table. 



Many porous carbonate rocks show evidence of both solution and 

 deposition. Presumably much of this development could have taken 

 place almost simultaneously, so that the solutions were at a point 

 of being slightly undersaturated at one moment with power to dissolve 

 and slightly oversaturated at another moment with resulting deposition. 

 The process of solution and deposition has taken place many times 



