116 



the beaker, after five minutes' rest, repeating these wash- 

 ings, ;iik1 examinations a1 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 deposited 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 100 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 pur- 

 pose, it is well to boil it for a few minutes in a rather strong- 

 solution of washing soda. 



Hardening of Fossils. Some of the more delicate fossils 

 from the shales require hardening, to preserve them perma- 

 nentlv. This may be done by warming the fossil and 

 then dipping it into hot thin glue, or the fossil may be well 

 soaked in a very thin solution of white shellac, and then laid 

 aside to dry. This latter method is preferable. 



To make Artificial Casts from Natural Molds. In many 

 cases the fossil has been removed in one way or another, and 

 nothing but the mold remains. In such cases a cast made 

 with gutta-percha will often give the surface features of the 

 fossil with even greater detail than could be seen on the orig- 

 inal specimen. A small piece of gutta-percha is to be soft- 

 ened in hot water, and pressed into the moistened mold with 

 the thumb, which must be wet, to prevent sticking. Consid- 

 erable pressure is required, and the squeezed-out borders 

 should be folded in again, in order to insure a perfect cast. 

 "Modeling composition for dental purposes, No. 2, medium," 

 is often better than gutta-percha. It is likewise made soft 

 by heating in hot water. 



* Woodward and Thomas, Geol. of 3Iinnesota, Final Report. Vol. III.: Pt. I.. pp. ■■!?> 

 and 86. 



