8i2 NORTH AMERICAN INDEX FOSSILS. 



repeating this five or six times. As in the first washing, mount and 

 examine a few sHdes from the material at the bottom of the bottle, 

 mounting and preserving slides, if found to be of value. If nothing 

 of value is found, pour out the contents of the bottle and fill up 

 again as before from the beaker, after five minutes' rest, repeating 

 these washings, and examinations at shorter resting intervals, of, 

 say, three, two, and one minute, or less, until nothing but the 

 coarsest sand remains in the beaker. . . . Each layer of clay, as de- 

 posited by its specific gravity, has now been examined, and most of 

 the fossils are contained in some one, or possibly two, of them. 

 Nineteen twentieths of the original sample of clay have been 

 washed away and in the selected one twentieth that remains there 

 may be one fair fossil to lOO grains of sand."^ 



In the above process, all glassware, etc., must be perfectly clean, 

 and the water used must be first filtered, otherwise organisms 

 foreign to the rock under investigation may appear. In the final 

 disintegration of the shale for this purpose, it is well to boil it for 

 a few minutes in a rather strong solution of washing soda. 



(b) Chemical Methods. — Fossils in a calcareous matrix may be 

 developed by dripping rain water or water charged with carbon 

 dioxide. The matrix of lime-mud or lime-sand generally dissolves 

 more readily than the organism. Boiling of the specimen in a 

 strong solution of sugar has also been advocated (Bather). This 

 attacks the amorphous limestone, but not the crystalline fossil. 

 The specimen should be removed from time to time, washed, dried, 

 and brushed. The fossil, if partly exposed, may be painted over 

 with a protective solution such as an alcoholic solution of/' Brillac," 

 or of pure shellac, and the specimen then suspended in weak hydro- 

 chloric, or in acetic acid.* 



" After a period varying from half an hour to twenty-four 

 hours, according to the nature of the matrix, the specimen is taken 

 out, washed in pure water, and allowed to dry. The softened 

 matrix is then removed with a brush of bristle or horse-hair. Any 

 freshly exposed portions of the fossil are then coated with the 



^ Woodward and Thomas, Geol. of Minnesota, Final Report, Vol. III., Pt. I., pp. 

 25 and 26. 



* Bather recommends hypo-acetine, a preparation sold by Tyror & Co., Ltd., 

 Sterling Chemical Works, Stratford, London, because it will partly dissolve and 

 partly soften the matrix. 



