356 



Subsurface Geologic Methods 



TABLE 20 



Tentative Scale of Crystal Sizes in Dolomites and Limestones 



Crystal Diameter 



Invisible 



Less than 10 mm. 



Less than 0.02 mm. 



0.02 to 0.1 mm. 



0.1 to 2.0 mm. 



2.0 to 10 mm. 



Descriptive Adjectives 



1. Mat 



2. Microcrystalline 



a. Cryptocrystalline 



b. Finely crystalline 



c. Mediocrystalline 



d. Coarsely crystalline 



3. Megacrystalline 



Definitions 

 Mat. — Compact, exceptionally homogeneous; having a dull but even surface under low binocular micro- 

 scope; resembling limestones used in lithography; as, a mat limestone, a mat dolomite. 

 Microcrystalline. — Having crystals less than 10 m.-n. long; having crystals small enough to be viewed under 

 low-power binocular microscope. 



Cryptocrystalline.— \adi&Xiacl\y crystalline; showing very small, indistinct crystal faces; composed of 

 crystals too small to be measured under low-power binocular microscope. 



Megacrystalline. — Having crystals 10 mm. or more in length; having crystals too large to be readily dis- 

 cernible under low-power binocular microscope. 



Porosity, Permeability, and Oil Stain 



Description of the porosity, permeability, and oil stain of cuttings 

 samples is one of the most important parts of the sample examiner's work. 

 In sandstone, the porosity is determined by the size of the grain, the 

 sorting, and the amount of the cement present. The grain size should be 

 described by some standard scale such as Wentworth's ^ or Alling's.^ 



Figure 148. Isolated pin-point porosity. Reverse circulated cuttings. x5. Penrose's 

 University 2, sec. 3, blk. 10, University lands, Andrews County, Texas; 4410- 

 4415 feet. 



* Wentworth, C. K., A Scale of Grade and Cl-ass Terms for Clastic Sediments: Jour. GeoL, vol. 30, 

 pp. 377-392, 1922. 



"Ailing, H. L., A Metric Grade Scale for Sedimentary Rocks: Jour. Geol., vol. 51, pp. 259-269, 1943. 



