solution. Watch the reactions to ascertain that the C0 2 is coming from the 

 solid chip. 



Acid reactions are unreliable when the test chips are wet with water before 

 they are introduced into the acid. The film of water on the chip and water 

 filling the pores prevents immediate contact of the acid with the rock. 



Calcareous shales and sandstones sometimes react as violently as pure 

 limestones. One should check the reacting chip from time to time for insoluble 

 residues. Dolomitic shales and sandstones, likewise, may react like dolomite. 



Weak, partially spent acid gives correspondingly weak reactions. One 

 should keep near at hand a few small test chips of carbonate whose reactions in 

 acid of the right strength are known. The acid should be checked periodically 

 by means of these test chips. Fresh acid should be obtained frequently when 

 one is working largely with carbonate rocks. 



A number of stain tests for differentiating dolomites and calcitic lime- 

 stones are applicable only to very light-colored rocks, and all are more effective 

 on light colors. Stain tests are very good for determining the manner of dis- 

 tribution of dolomite and calcite grains or crystals in the rocks. The method 

 given below is quite easy to use, since it does not require hot solutions. It is 

 known as the Fairbanks method. 



Mix 0.24 gram of haematoxylin and 1.6 grams of aluminum chloride in 

 22 cubic centimeters of water. Bring the solution to a boil, and then allow it 

 to cool. Then add a small quantity of hydrogen peroxide, and filter. Keep the 

 solution in a dark glass bottle. 



To test a specimen in Fairbanks solution, immerse it for a few seconds 

 and then rinse it immediately in clean water. Do not use a strong stream. The 

 calcite will take on a deep purple color, while the dolomite will be unaffected. 

 Beware of finely porous or sucrosic rocks, for some of the stain may lodge in 

 the interstices and give the appeance of staining. When staining thin or 

 polished sections, first etch them slightly in acetic or hydrochloric acid. Etch 

 only sufficiently to produce a dull surface on the calcite. Wash immediately in 

 water and dry thoroughly before applying the stain. 



Grain Size: The grains of carbonate rocks vary greatly in size and general 

 appearance in short lateral and vertical distances. It is necessary to describe 

 the grains completely in order to determine the origin of the rock. The grains 

 may be fragments of limestone, shells, microfossils, oolites, algal remains, 

 crystals or precipitated grains, or any combination of these types. Grain sizes 

 are measured by means of diaphram scales and grids, or grids placed in the 

 scoop tray. The latter are not entirely satisfactory because of the difficulty of 

 breaking down the carbonate rocks to individual grains. 



The grain-size classification given in the following table is a modification 

 and simplification of various classifications occasionally used in some institu- 



33 



